diff --git a/lib/yunohost/app.py b/lib/yunohost/app.py
index 33188e27..1cb2e609 100644
--- a/lib/yunohost/app.py
+++ b/lib/yunohost/app.py
@@ -33,6 +33,7 @@ import time
 import re
 import socket
 import urlparse
+import errno
 
 from moulinette.helpers import win_msg, random_password, is_true, validate
 from moulinette.core import MoulinetteError
@@ -55,7 +56,7 @@ def app_listlists():
             if '.json' in filename:
                 list_list.append(filename[:len(filename)-5])
     except OSError:
-        raise MoulinetteError(1, _("No list found"))
+        raise MoulinetteError(1, m18n.n('no_list_found'))
 
     return { 'Lists' : list_list }
 
@@ -77,12 +78,14 @@ def app_fetchlist(url=None, name=None):
         url = 'http://app.yunohost.org/list.json'
         name = 'yunohost'
     else:
-        if name is None: raise MoulinetteError(22, _("You must indicate a name for your custom list"))
+        if name is None:
+            raise MoulinetteError(errno.EINVAL,
+                                  m18n.n('custom_list_name_required'))
 
     list_file = '%s/%s.json' % (repo_path, name)
     if os.system('wget "%s" -O "%s.tmp"' % (url, list_file)) != 0:
         os.remove('%s.tmp' % list_file)
-        raise MoulinetteError(1, _("List server connection failed"))
+        raise MoulinetteError(errno.EBADR, m18n.n('list_retrieve_error'))
 
     # Rename fetched temp list
     os.rename('%s.tmp' % list_file, list_file)
@@ -90,7 +93,7 @@ def app_fetchlist(url=None, name=None):
     os.system("touch /etc/cron.d/yunohost-applist-%s" % name)
     os.system("echo '00 00 * * * root yunohost app fetchlist -u %s -n %s --no-ldap > /dev/null 2>&1' >/etc/cron.d/yunohost-applist-%s" % (url, name, name))
 
-    msignals.display(_("List successfully fetched"), 'success')
+    msignals.display(m18n.n('list_fetched'), 'success')
 
 
 def app_removelist(name):
@@ -105,9 +108,9 @@ def app_removelist(name):
         os.remove('%s/%s.json' % (repo_path, name))
         os.remove("/etc/cron.d/yunohost-applist-%s" % name)
     except OSError:
-        raise MoulinetteError(22, _("Unknown list"))
+        raise MoulinetteError(errno.ENOENT, m18n.n('unknown_list'))
 
-    msignals.display(_("List successfully removed"), 'success')
+    msignals.display(m18n.n('list_removed'), 'success')
 
 
 def app_list(offset=None, limit=None, filter=None, raw=False):
@@ -268,7 +271,7 @@ def app_upgrade(auth, app, url=None, file=None):
     try:
         app_list()
     except MoulinetteError:
-        raise MoulinetteError(1, _("No app to upgrade"))
+        raise MoulinetteError(errno.ENODATA, m18n.n('app_no_upgrade'))
 
     upgraded_apps = []
 
@@ -281,7 +284,8 @@ def app_upgrade(auth, app, url=None, file=None):
     for app_id in app:
         installed = _is_installed(app_id)
         if not installed:
-            raise MoulinetteError(1, _("%s is not installed") % app_id)
+            raise MoulinetteError(errno.ENOPKG,
+                                  m18n.n('app_not_installed') % app_id)
 
         if app_id in upgraded_apps:
             continue
@@ -299,7 +303,8 @@ def app_upgrade(auth, app, url=None, file=None):
         elif url:
             manifest = _fetch_app_from_git(url)
         elif 'lastUpdate' not in new_app_dict or 'git' not in new_app_dict:
-            raise MoulinetteError(22, _("%s is a custom app, please provide an URL manually in order to upgrade it") % app_id)
+            raise MoulinetteError(errno.EDESTADDRREQ,
+                                  m18n.n('custom_app_url_required') % app_id)
         elif (new_app_dict['lastUpdate'] > current_app_dict['lastUpdate']) \
               or ('update_time' not in current_app_dict['settings'] \
                    and (new_app_dict['lastUpdate'] > current_app_dict['settings']['install_time'])) \
@@ -311,7 +316,8 @@ def app_upgrade(auth, app, url=None, file=None):
 
         # Check min version
         if 'min_version' in manifest and __version__ < manifest['min_version']:
-            raise MoulinetteError(1, _("%s requires a more recent version of the moulinette") % app_id)
+            raise MoulinetteError(errno.EPERM,
+                                  m18n.n('app_recent_version_required') % app_id)
 
         app_setting_path = apps_setting_path +'/'+ app_id
 
@@ -355,12 +361,12 @@ def app_upgrade(auth, app, url=None, file=None):
 
         # So much win
         upgraded_apps.append(app_id)
-        msignals.display(_("%s upgraded successfully") % app_id, 'success')
+        msignals.display(m18n.n('app_upgraded') % app_id, 'success')
 
     if not upgraded_apps:
-        raise MoulinetteError(1, _("No app to upgrade"))
+        raise MoulinetteError(errno.ENODATA, m18n.n('no_app_upgrade'))
 
-    msignals.display(_("Upgrade complete"), 'success')
+    msignals.display(m18n.n('upgrade_complete'), 'success')
 
 
 def app_install(auth, app, label=None, args=None):
@@ -386,19 +392,21 @@ def app_install(auth, app, label=None, args=None):
 
     # Check ID
     if 'id' not in manifest or '__' in manifest['id']:
-        raise MoulinetteError(22, _("App id is invalid"))
+        raise MoulinetteError(errno.EINVAL, m18n.n('app_id_invalid'))
 
     app_id = manifest['id']
 
     # Check min version
     if 'min_version' in manifest and __version__ < manifest['min_version']:
-        raise MoulinetteError(1, _("%s requires a more recent version of the moulinette") % app_id)
+        raise MoulinetteError(errno.EPERM,
+                              m18n.n('app_recent_version_required') % app_id)
 
     # Check if app can be forked
     instance_number = _installed_instance_number(app_id, last=True) + 1
     if instance_number > 1 :
         if 'multi_instance' not in manifest or not is_true(manifest['multi_instance']):
-            raise MoulinetteError(1, _("App is already installed"))
+            raise MoulinetteError(errno.EEXIST,
+                                  m18n.n('app_already_installed') % app_id)
 
         app_id_forked = app_id + '__' + str(instance_number)
 
@@ -467,18 +475,18 @@ def app_install(auth, app, label=None, args=None):
             os.system('chown -R root: %s' % app_setting_path)
             os.system('chown -R admin: %s/scripts' % app_setting_path)
             app_ssowatconf(auth)
-            msignals.display(_("Installation complete"), 'success')
+            msignals.display(m18n.n('installation_complete'), 'success')
         else:
             #TODO: display script fail messages
             hook_remove(app_id)
             shutil.rmtree(app_setting_path)
             shutil.rmtree(app_tmp_folder)
-            raise MoulinetteError(1, _("Installation failed"))
+            raise MoulinetteError(errno.EIO, m18n.n('installation_failed'))
     except KeyboardInterrupt, EOFError:
         hook_remove(app_id)
         shutil.rmtree(app_setting_path)
         shutil.rmtree(app_tmp_folder)
-        raise MoulinetteError(125, _("Interrupted"))
+        raise MoulinetteError(errno.EINTR, m18n.g('operation_interrupted'))
 
 
 def app_remove(app):
@@ -492,7 +500,7 @@ def app_remove(app):
     from yunohost.hook import hook_exec, hook_remove
 
     if not _is_installed(app):
-        raise MoulinetteError(22, _("App is not installed"))
+        raise MoulinetteError(errno.EINVAL, m18n.n('app_not_installed') % app)
 
     app_setting_path = apps_setting_path + app
 
@@ -512,7 +520,7 @@ def app_remove(app):
     shutil.rmtree('/tmp/yunohost_remove')
     hook_remove(app)
     app_ssowatconf()
-    msignals.display(_("App removed: %s") % app, 'success')
+    msignals.display(m18n.n('app_removed') % app, 'success')
 
 
 def app_addaccess(auth, apps, users):
@@ -536,7 +544,8 @@ def app_addaccess(auth, apps, users):
 
     for app in apps:
         if not _is_installed(app):
-            raise MoulinetteError(22, _("App is not installed"))
+            raise MoulinetteError(errno.EINVAL,
+                                  m18n.n('app_not_installed') % app)
 
         with open(apps_setting_path + app +'/settings.yml') as f:
             app_settings = yaml.load(f)
@@ -589,7 +598,8 @@ def app_removeaccess(auth, apps, users):
         new_users = ''
 
         if not _is_installed(app):
-            raise MoulinetteError(22, _("App is not installed"))
+            raise MoulinetteError(errno.EINVAL,
+                                  m18n.n('app_not_installed') % app)
 
         with open(apps_setting_path + app +'/settings.yml') as f:
             app_settings = yaml.load(f)
@@ -631,7 +641,8 @@ def app_clearaccess(auth, apps):
 
     for app in apps:
         if not _is_installed(app):
-            raise MoulinetteError(22, _("App is not installed"))
+            raise MoulinetteError(errno.EINVAL,
+                                  m18n.n('app_not_installed') % app)
 
         with open(apps_setting_path + app +'/settings.yml') as f:
             app_settings = yaml.load(f)
@@ -654,7 +665,7 @@ def app_makedefault(app, domain=None):
 
     """
     if not _is_installed(app):
-        raise MoulinetteError(22, _("App is not installed"))
+        raise MoulinetteError(errno.EINVAL, m18n.n('app_not_installed') % app)
 
     with open(apps_setting_path + app +'/settings.yml') as f:
         app_settings = yaml.load(f)
@@ -665,10 +676,11 @@ def app_makedefault(app, domain=None):
     if domain is None:
         domain = app_domain
     elif domain not in domain_list()['Domains']:
-        raise MoulinetteError(22, _("Domain doesn't exists"))
+        raise MoulinetteError(errno.EINVAL, m18n.n('domain_unknown'))
 
     if '/' in app_map(raw=True)[domain]:
-        raise MoulinetteError(1, _("An app is already installed on this location"))
+        raise MoulinetteError(errno.EEXIST,
+                              m18n.n('app_location_already_used'))
 
     try:
         with open('/etc/ssowat/conf.json.persistent') as json_conf:
@@ -686,7 +698,7 @@ def app_makedefault(app, domain=None):
 
     os.system('chmod 644 /etc/ssowat/conf.json.persistent')
 
-    win_msg('SSOwat persistent configuration has been updated')
+    msignals.display(m18n.n('ssowat_conf_updated'), 'success')
 
 
 
@@ -780,10 +792,10 @@ def app_checkport(port):
         s.connect(("localhost", int(port)))
         s.close()
     except socket.error:
-        msignals.display(_("Port available: %s") % str(port), 'success')
+        msignals.display(m18n.n('port_available') % int(port), 'success')
     else:
-        raise MoulinetteError(22, _("Port not available: %s") % str(port))
-
+        raise MoulinetteError(errno.EINVAL,
+                              m18n.n('port_unavailable') % int(port))
 
 
 def app_checkurl(auth, url, app=None):
@@ -815,14 +827,15 @@ def app_checkurl(auth, url, app=None):
     validate(r'^([a-zA-Z0-9]{1}([a-zA-Z0-9\-]*[a-zA-Z0-9])*)(\.[a-zA-Z0-9]{1}([a-zA-Z0-9\-]*[a-zA-Z0-9])*)*(\.[a-zA-Z]{1}([a-zA-Z0-9\-]*[a-zA-Z0-9])*)$', domain)
 
     if domain not in domain_list(auth)['domains']:
-        raise MoulinetteError(22, _("Domain doesn't exists"))
+        raise MoulinetteError(errno.EINVAL, m18n.n('domain_unknown'))
 
     if domain in apps_map:
         if path in apps_map[domain]:
-            raise MoulinetteError(1, _("An app is already installed on this location"))
+            raise MoulinetteError(errno.EINVAL, m18n.n('app_location_already_used'))
         for app_path, v in apps_map[domain].items():
             if app_path in path and app_path.count('/') < path.count('/'):
-                raise MoulinetteError(1, _("Unable to install app at this location"))
+                raise MoulinetteError(errno.EPERM,
+                                      m18n.n('app_location_install_failed'))
 
     if app is not None:
         app_setting(app, 'domain', value=domain)
@@ -852,13 +865,13 @@ def app_initdb(user, password=None, db=None, sql=None):
     mysql_root_pwd = open('/etc/yunohost/mysql').read().rstrip()
     mysql_command = 'mysql -u root -p%s -e "CREATE DATABASE %s ; GRANT ALL PRIVILEGES ON %s.* TO \'%s\'@localhost IDENTIFIED BY \'%s\';"' % (mysql_root_pwd, db, db, user, password)
     if os.system(mysql_command) != 0:
-        raise MoulinetteError(1, _("MySQL DB creation failed"))
+        raise MoulinetteError(errno.EIO, m18n.n('mysql_db_creation_failed'))
     if sql is not None:
         if os.system('mysql -u %s -p%s %s < %s' % (user, password, db, sql)) != 0:
-            raise MoulinetteError(1, _("MySQL DB init failed"))
+            raise MoulinetteError(errno.EIO, m18n.n('mysql_db_init_failed'))
 
     if not return_pwd:
-        msignals.display(_("Database initiliazed"), 'success')
+        msignals.display(m18n.n('mysql_db_initialized'), 'success')
 
 
 def app_ssowatconf(auth):
@@ -951,7 +964,7 @@ def app_ssowatconf(auth):
     with open('/etc/ssowat/conf.json', 'w+') as f:
         json.dump(conf_dict, f, sort_keys=True, indent=4)
 
-    msignals.display(_('SSOwat configuration generated'), 'success')
+    msignals.display(m18n.n('ssowat_conf_generated'), 'success')
 
 
 def _extract_app_from_file(path, remove=False):
@@ -968,7 +981,7 @@ def _extract_app_from_file(path, remove=False):
     """
     global app_tmp_folder
 
-    print(_('Extracting...'))
+    msignals.display(m18n.n('extracting'))
 
     if os.path.exists(app_tmp_folder): shutil.rmtree(app_tmp_folder)
     os.makedirs(app_tmp_folder)
@@ -988,7 +1001,7 @@ def _extract_app_from_file(path, remove=False):
         extract_result = 1
 
     if extract_result != 0:
-        raise MoulinetteError(22, _("Invalid install file"))
+        raise MoulinetteError(errno.EINVAL, m18n.n('app_extraction_failed'))
 
     try:
         if len(os.listdir(app_tmp_folder)) == 1:
@@ -998,9 +1011,9 @@ def _extract_app_from_file(path, remove=False):
             manifest = json.loads(str(json_manifest.read()))
             manifest['lastUpdate'] = int(time.time())
     except IOError:
-        raise MoulinetteError(1, _("Invalid App file"))
+        raise MoulinetteError(errno.EIO, m18n.n('app_install_files_invalid'))
 
-    print(_('OK'))
+    msignals.display(m18n.n('done'))
 
     return manifest
 
@@ -1018,7 +1031,7 @@ def _fetch_app_from_git(app):
     """
     global app_tmp_folder
 
-    print(_('Downloading...'))
+    msignals.display(m18n.n('downloading'))
 
     if ('@' in app) or ('http://' in app) or ('https://' in app):
         if "github.com" in app:
@@ -1036,7 +1049,7 @@ def _fetch_app_from_git(app):
                 manifest = json.loads(str(json_manifest.read()))
                 manifest['lastUpdate'] = int(time.time())
         except IOError:
-            raise MoulinetteError(1, _("Invalid App manifest"))
+            raise MoulinetteError(errno.EIO, m18n.n('app_manifest_invalid'))
 
     else:
         app_dict = app_list(raw=True)
@@ -1046,7 +1059,7 @@ def _fetch_app_from_git(app):
             app_info['manifest']['lastUpdate'] = app_info['lastUpdate']
             manifest = app_info['manifest']
         else:
-            raise MoulinetteError(22, _("App doesn't exists"))
+            raise MoulinetteError(errno.EINVAL, m18n.n('app_unknown'))
 
         if "github.com" in app_info['git']['url']:
             url = app_info['git']['url'].replace("git@github.com:", "https://github.com/")
@@ -1063,9 +1076,9 @@ def _fetch_app_from_git(app):
         git_result_2 = os.system('cd %s && git reset --hard %s' % (app_tmp_folder, str(app_info['git']['revision'])))
 
     if not git_result == git_result_2 == 0:
-        raise MoulinetteError(22, _("Sources fetching failed"))
+        raise MoulinetteError(errno.EIO, m18n.n('app_sources_fetch_failed'))
 
-    print(_('OK'))
+    msignals.display(m18n.n('done'))
 
     return manifest
 
@@ -1134,4 +1147,3 @@ def _is_installed(app):
             continue
 
     return False
-
diff --git a/lib/yunohost/domain.py b/lib/yunohost/domain.py
index d9dd0626..5faa37ac 100644
--- a/lib/yunohost/domain.py
+++ b/lib/yunohost/domain.py
@@ -30,6 +30,7 @@ import re
 import shutil
 import json
 import yaml
+import errno
 from urllib import urlopen
 
 from moulinette.core import MoulinetteError
@@ -89,7 +90,7 @@ def domain_add(auth, domains, main=False, dyndns=False):
         # DynDNS domain
         if dyndns:
             if len(domain.split('.')) < 3:
-                raise MoulinetteError(22, _("Invalid domain '%s' for DynDNS" % domain))
+                raise MoulinetteError(errno.EINVAL, m18n.n('domain_dyndns_invalid'))
             import requests
             from yunohost.dyndns import dyndns_subscribe
 
@@ -98,10 +99,12 @@ def domain_add(auth, domains, main=False, dyndns=False):
             dyndomain  = '.'.join(domain.split('.')[1:])
             if dyndomain in dyndomains:
                 if os.path.exists('/etc/cron.d/yunohost-dyndns'):
-                    raise MoulinetteError(22, _("You already have a DynDNS domain"))
+                    raise MoulinetteError(errno.EPERM,
+                                          m18n.n('domain_dyndns_already_subscribed'))
                 dyndns_subscribe(domain=domain)
             else:
-                raise MoulinetteError(22, _("Unknown DynDNS domain '%s'" % dyndomain))
+                raise MoulinetteError(errno.EINVAL,
+                                      m18n.n('domain_dyndns_root_unknown'))
 
         # Commands
         ssl_dir = '/usr/share/yunohost/yunohost-config/ssl/yunoCA'
@@ -131,12 +134,13 @@ def domain_add(auth, domains, main=False, dyndns=False):
 
         for command in command_list:
             if os.system(command) != 0:
-                raise MoulinetteError(17, _("An error occurred during certificate generation"))
+                raise MoulinetteError(errno.EIO,
+                                      m18n.n('domain_cert_gen_failed'))
 
         try:
             auth.validate_uniqueness({ 'virtualdomain': domain })
         except MoulinetteError:
-            raise MoulinetteError(17, _("Domain already created"))
+            raise MoulinetteError(errno.EEXIST, m18n.n('domain_exists'))
 
 
         attr_dict['virtualdomain'] = domain
@@ -169,7 +173,8 @@ def domain_add(auth, domains, main=False, dyndns=False):
             os.system('chown bind /var/lib/bind/%s.zone' % domain)
 
         else:
-            raise MoulinetteError(17, _("Zone file already exists for %s") % domain)
+            raise MoulinetteError(errno.EEXIST,
+                                  m18n.n('domain_zone_exists'))
 
         conf_lines = [
             'zone "%s" {' % domain,
@@ -228,12 +233,12 @@ def domain_add(auth, domains, main=False, dyndns=False):
             result.append(domain)
             continue
         else:
-            raise MoulinetteError(169, _("An error occurred during domain creation"))
+            raise MoulinetteError(errno.EIO, m18n.n('domain_creation_failed'))
 
 
     os.system('yunohost app ssowatconf > /dev/null 2>&1')
 
-    msignals.display(_("Domain(s) successfully created."), 'success')
+    msignals.display(m18n.n('domain_created'), 'success')
     return { 'domains': result }
 
 
@@ -253,7 +258,7 @@ def domain_remove(auth, domains):
 
     for domain in domains:
         if domain not in domains_list:
-            raise MoulinetteError(22, _("Unknown domain '%s'") % domain)
+            raise MoulinetteError(errno.EINVAL, m18n.n('domain_unknown'))
 
         # Check if apps are installed on the domain
         for app in os.listdir('/etc/yunohost/apps/'):
@@ -264,7 +269,8 @@ def domain_remove(auth, domains):
                     continue
                 else:
                     if app_domain == domain:
-                        raise MoulinetteError(1, _("One or more apps are installed on this domain, please uninstall them before proceed to domain removal"))
+                        raise MoulinetteError(errno.EPERM,
+                                              m18n.n('domain_uninstall_app_first'))
 
         if auth.remove('virtualdomain=' + domain + ',ou=domains'):
             try:
@@ -291,13 +297,12 @@ def domain_remove(auth, domains):
             result.append(domain)
             continue
         else:
-            raise MoulinetteError(169, _("An error occurred during domain deletion"))
+            raise MoulinetteError(errno.EIO, m18n.n('domain_deletion_failed'))
 
     os.system('yunohost app ssowatconf > /dev/null 2>&1')
     os.system('service nginx reload')
     os.system('service bind9 reload')
     os.system('service metronome restart')
 
-    msignals.display(_("Domain(s) successfully deleted."), 'success')
+    msignals.display(m18n.n('domain_deleted'), 'success')
     return { 'domains': result }
-
diff --git a/lib/yunohost/dyndns.py b/lib/yunohost/dyndns.py
index 5525a8e3..08377753 100644
--- a/lib/yunohost/dyndns.py
+++ b/lib/yunohost/dyndns.py
@@ -29,6 +29,7 @@ import requests
 import json
 import glob
 import base64
+import errno
 
 from moulinette.core import MoulinetteError
 
@@ -49,7 +50,7 @@ def dyndns_subscribe(subscribe_host="dyndns.yunohost.org", domain=None, key=None
 
     # Verify if domain is available
     if requests.get('http://%s/test/%s' % (subscribe_host, domain)).status_code != 200:
-        raise MoulinetteError(17, _("DynDNS domain is already taken"))
+        raise MoulinetteError(errno.EEXIST, m18n.n('dyndns_unavailable'))
 
     if key is None:
         if len(glob.glob('/etc/yunohost/dyndns/*.key')) == 0:
@@ -67,9 +68,10 @@ def dyndns_subscribe(subscribe_host="dyndns.yunohost.org", domain=None, key=None
     if r.status_code != 201:
         try:    error = json.loads(r.text)['error']
         except: error = "Server error"
-        raise MoulinetteError(1, _("An error occurred during DynDNS registration: %s") % error)
+        raise MoulinetteError(errno.EPERM,
+                              m18n.n('dyndns_registration_failed') % error)
 
-    msignals.display(_("Subscribed to DynDNS."), 'success')
+    msignals.display(m18n.n('dyndns_registered'), 'success')
 
     dyndns_installcron()
 
@@ -134,12 +136,13 @@ def dyndns_update(dyn_host="dynhost.yunohost.org", domain=None, key=None, ip=Non
         else:
             private_key_file = key
         if os.system('/usr/bin/nsupdate -k %s /etc/yunohost/dyndns/zone' % private_key_file) == 0:
-            msignals.display(_("IP successfully updated."), 'success')
+            msignals.display(m18n.n('dyndns_ip_updated'), 'success')
             with open('/etc/yunohost/dyndns/old_ip', 'w') as f:
                 f.write(new_ip)
         else:
             os.system('rm /etc/yunohost/dyndns/old_ip > /dev/null 2>&1')
-            raise MoulinetteError(1, _("An error occurred during DynDNS update"))
+            raise MoulinetteError(errno.EPERM,
+                                  m18n.n('dyndns_ip_update_failed'))
 
 
 def dyndns_installcron():
@@ -151,7 +154,7 @@ def dyndns_installcron():
     with open('/etc/cron.d/yunohost-dyndns', 'w+') as f:
         f.write('*/2 * * * * root yunohost dyndns update >> /dev/null')
 
-    msignals.display(_("DynDNS cron installed."), 'success')
+    msignals.display(m18n.n('dyndns_cron_installed'), 'success')
 
 
 def dyndns_removecron():
@@ -163,6 +166,6 @@ def dyndns_removecron():
     try:
         os.remove("/etc/cron.d/yunohost-dyndns")
     except:
-        raise MoulinetteError(167,_("DynDNS cron was not installed"))
+        raise MoulinetteError(errno.EIO, m18n.n('dyndns_cron_remove_failed'))
 
-    msignals.display(_("DynDNS cron removed."), 'success')
+    msignals.display(m18n.n('dyndns_cron_removed'), 'success')
diff --git a/lib/yunohost/firewall.py b/lib/yunohost/firewall.py
index 68089c94..b5c2a478 100644
--- a/lib/yunohost/firewall.py
+++ b/lib/yunohost/firewall.py
@@ -26,6 +26,7 @@
 import os
 import sys
 import yaml
+import errno
 try:
     import miniupnpc
 except ImportError:
@@ -66,7 +67,7 @@ def firewall_allow(port=None, protocol='TCP', ipv6=False, no_upnp=False):
         if port not in firewall[ipv][protocol]:
             firewall[ipv][protocol].append(port)
         else:
-            msignals.display(_("Port already openned: %d" % port), 'warning')
+            msignals.display(m18n.n('port_already_opened') % port, 'warning')
 
     with open('/etc/yunohost/firewall.yml', 'w') as f:
         yaml.safe_dump(firewall, f, default_flow_style=False)
@@ -102,7 +103,7 @@ def firewall_disallow(port=None, protocol='TCP', ipv6=False):
         if port in firewall[ipv][protocol]:
             firewall[ipv][protocol].remove(port)
         else:
-            msignals.display(_("Port already closed: %d" % port), 'warning')
+            msignals.display(m18n.n('port_already_closed') % port, 'warning')
 
     with open('/etc/yunohost/firewall.yml', 'w') as f:
         yaml.safe_dump(firewall, f, default_flow_style=False)
@@ -140,7 +141,7 @@ def firewall_reload():
 
     # IPv4
     if os.system("iptables -P INPUT ACCEPT") != 0:
-        raise MoulinetteError(1, _("You cannot play with iptables here. You are either in a container or your kernel does not support it."))
+        raise MoulinetteError(errno.ESRCH, m18n.n('iptables_unavailable'))
     if upnp:
         try:
             upnpc = miniupnpc.UPnP()
@@ -154,9 +155,9 @@ def firewall_reload():
                             except: pass
                         upnpc.addportmapping(port, protocol, upnpc.lanaddr, port, 'yunohost firewall : port %d' % port, '')
             else:
-                raise MoulinetteError(1, _("No uPnP device found"))
+                raise MoulinetteError(errno.ENXIO, m18n.n('upnp_dev_not_found'))
         except:
-            msignals.display(_("An error occured during uPnP port openning"), 'warning')
+            msignals.display(m18n.n('upnp_port_open_failed'), 'warning')
 
     os.system("iptables -F")
     os.system("iptables -X")
@@ -196,7 +197,7 @@ def firewall_reload():
         os.system("ip6tables -P INPUT DROP")
 
     os.system("service fail2ban restart")
-    msignals.display(_("Firewall successfully reloaded"), 'success')
+    msignals.display(m18n.n('firewall_reloaded'), 'success')
 
     return firewall_list()
 
@@ -209,7 +210,6 @@ def firewall_upnp(action=None):
         action -- enable/disable
 
     """
-
     firewall = firewall_list(raw=True)
 
     if action:
@@ -221,7 +221,7 @@ def firewall_upnp(action=None):
         with open('/etc/cron.d/yunohost-firewall', 'w+') as f:
             f.write('*/50 * * * * root PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin yunohost firewall reload >>/dev/null')
 
-        msignals.display(_("uPnP successfully enabled"), 'success')
+        msignals.display(m18n.n('upnp_enabled'), 'success')
 
     if action == 'disable':
         firewall['uPnP']['enabled'] = False
@@ -242,7 +242,7 @@ def firewall_upnp(action=None):
         try: os.remove('/etc/cron.d/yunohost-firewall')
         except: pass
 
-        msignals.display(_("uPnP successfully disabled"), 'success')
+        msignals.display(m18n.n('upnp_disabled'), 'success')
 
     if action:
         os.system("cp /etc/yunohost/firewall.yml /etc/yunohost/firewall.yml.old")
@@ -260,7 +260,7 @@ def firewall_stop():
     """
 
     if os.system("iptables -P INPUT ACCEPT") != 0:
-        raise MoulinetteError(1, _("You cannot play with iptables here. You are either in a container or your kernel does not support it."))
+        raise MoulinetteError(errno.ESRCH, m18n.n('iptables_unavailable'))
 
     os.system("iptables -F")
     os.system("iptables -X")
diff --git a/lib/yunohost/hook.py b/lib/yunohost/hook.py
index b47dba0d..9c020bf9 100644
--- a/lib/yunohost/hook.py
+++ b/lib/yunohost/hook.py
@@ -27,6 +27,7 @@ import os
 import sys
 import re
 import json
+import errno
 
 from moulinette.helpers import colorize
 from moulinette.core import MoulinetteError
@@ -111,7 +112,7 @@ def hook_check(file):
         with open(file[:file.index('scripts/')] + 'manifest.json') as f:
             manifest = json.loads(str(f.read()))
     except:
-        raise MoulinetteError(22, _("Invalid app package"))
+        raise MoulinetteError(errno.EIO, m18n.n('app_manifest_invalid'))
 
     action = file[file.index('scripts/') + 8:]
     if 'arguments' in manifest and action in manifest['arguments']:
@@ -140,7 +141,9 @@ def hook_exec(file, args=None):
         for arg in required_args:
             if arg['name'] in args:
                 if 'choices' in arg and args[arg['name']] not in arg['choices']:
-                    raise MoulinetteError(22, _("Invalid choice") + ': ' + args[arg['name']])
+                    raise MoulinetteError(errno.EINVAL,
+                                          m18n.n('hook_choice_invalid')
+                                                  % args[arg['name']])
                 arg_list.append(args[arg['name']])
             else:
                 if os.isatty(1) and 'ask' in arg:
@@ -159,7 +162,9 @@ def hook_exec(file, args=None):
                 elif 'default' in arg:
                     arg_list.append(arg['default'])
                 else:
-                    raise MoulinetteError(22, _("Missing argument : %s") % arg['name'])
+                    raise MoulinetteError(errno.EINVAL,
+                                          m18n.n('hook_argument_missing')
+                                                  % arg['name'])
 
     file_path = "./"
     if "/" in file and file[0:2] != file_path:
diff --git a/lib/yunohost/locales/en.json b/lib/yunohost/locales/en.json
new file mode 100644
index 00000000..40b1f9a4
--- /dev/null
+++ b/lib/yunohost/locales/en.json
@@ -0,0 +1,127 @@
+{
+    "yunohost" : "YunoHost",
+
+    "upgrade_complete" : "Upgrade complete",
+    "installation_complete" : "Installation complete",
+    "installation_failed" : "Installation failed",
+
+    "no_list_found" : "No list found",
+    "custom_list_name_required" : "You must provide a name for your custom list",
+    "list_retrieve_error" : "Unable to retrieve the remote list",
+    "list_feteched" : "List successfully fetched",
+    "list_unknown" : "Unknown list",
+    "list_removed" : "List successfully removed",
+    "app_unknown" : "Unknown app",
+    "app_no_upgrade" : "No app to upgrade",
+    "app_not_installed" : "%s is not installed",
+    "custom_app_url_required" : "You must provide an URL to upgrade your custom app %s",
+    "app_recent_version_required" : "%s requires a more recent version of the moulinette",
+    "app_upgraded" : "%s successfully upgraded",
+    "app_id_invalid" : "Invalid app id",
+    "app_already_installed" : "%s is already installed",
+    "app_removed" : "%s successfully removed",
+    "app_location_already_used" : "An app is already installed on this location",
+    "app_location_install_failed" : "Unable to install the app on this location",
+    "app_extraction_failed" : "Unable to extract installation files",
+    "app_install_files_invalid" : "Invalid installation files",
+    "app_manifest_invalid" : "Invalid app manifest",
+    "app_sources_fetch_failed" : "Unable to fetch sources files",
+    "ssowat_conf_updated" : "SSOwat persistent configuration successfully updated",
+    "ssowat_conf_generated" : "SSOwat configuration successfully generated",
+    "mysql_db_creation_failed" : "MySQL database creation failed",
+    "mysql_db_init_failed" : "MySQL database init failed",
+    "mysql_db_initialized" : "MySQL database successfully initialized",
+    "extracting" : "Extracting...",
+    "downloading" : "Downloading...",
+    "done" : "Done.",
+
+    "domain_unknown" : "Unknown domain",
+    "domain_dyndns_invalid" : "Invalid domain to use with DynDNS",
+    "domain_dyndns_already_subscribed" : "You already have subscribed to a DynDNS domain",
+    "domain_dyndns_root_unknown" : "Unknown DynDNS root domain",
+    "domain_cert_gen_failed" : "Unable to generate certificate",
+    "domain_exists" : "Domain already exists",
+    "domain_zone_exists" : "Zone file already exists",
+    "domain_creation_failed" : "Unable to create domain",
+    "domain_created" : "Domain successfully created",
+    "domain_uninstall_app_first" : "One or more apps are installed on this domain. Please uninstall them before proceed to domain removal.",
+    "domain_deletion_failed" : "Unable to delete domain",
+    "domain_deleted" : "Domain successfully deleted",
+
+    "dyndns_unavailable" : "Unavailable DynDNS subdomain",
+    "dyndns_registration_failed" : "Unable to register DynDNS domain: %s",
+    "dyndns_registered" : "DynDNS domain successfully registered",
+    "dyndns_ip_update_failed" : "Unable to update IP address on DynDNS",
+    "dyndns_ip_updated" : "IP address successfully updated on DynDNS",
+    "dyndns_cron_installed" : "DynDNS cron job successfully installed",
+    "dyndns_cron_remove_failed" : "Unable to remove DynDNS cron job",
+    "dyndns_cron_removed" : "DynDNS cron job successfully removed",
+
+    "port_available" : "Port %d is available",
+    "port_unavailable" : "Port %d is not available",
+    "port_already_opened" : "Port %d is already opened",
+    "port_already_closed" : "Port %d is already closed",
+    "iptables_unavailable" : "You cannot play with iptables here. You are either in a container or your kernel does not support it.",
+    "upnp_dev_not_found" : "No uPnP device found",
+    "upnp_port_open_failed" : "Unable to open uPnP ports",
+    "upnp_enabled" : "uPnP successfully enabled",
+    "upnp_disabled" : "uPnP successfully disabled",
+    "firewall_reloaded" : "Firewall successfully reloaded",
+
+    "hook_choice_invalid" : "Invalid choice '%s'",
+    "hook_argument_missing" : "Missing argument '%s'",
+
+    "mountpoint_unknown" : "Unknown mountpoint",
+    "unit_unknown" : "Unknown unit '%s'",
+    "monitor_period_invalid" : "Invalid time period",
+    "monitor_stats_no_update" : "No monitoring statistics to update",
+    "monitor_stats_file_not_found" : "Statistics file not found",
+    "monitor_stats_period_unavailable" : "No available statistics for the period",
+    "monitor_enabled" : "Server monitoring successfully enabled",
+    "monitor_disabled" : "Server monitoring successfully disabled",
+    "monitor_not_enabled" : "Server monitoring is not enabled",
+    "monitor_glances_con_failed" : "Unable to connect to Glances server",
+
+    "service_unknown" : "Unknown service '%s'",
+    "service_start_failed" : "Unable to start service '%s'",
+    "service_already_started" : "Service '%s' is already started",
+    "service_started" : "Service '%s' successfully started",
+    "service_stop_failed" : "Unable to stop service '%s'",
+    "service_already_stopped" : "Service '%s' is already stopped",
+    "service_stopped" : "Service '%s' successfully stopped",
+    "service_enable_failed" : "Unable to enable service '%s'",
+    "service_enabled" : "Service '%s' successfully enabled",
+    "service_disable_failed" : "Unable to disable service '%s'",
+    "service_disabled" : "Service '%s' successfully disabled",
+    "service_status_failed" : "Unable to determine status of service '%s'",
+    "service_no_log" : "No log to display for service '%s'",
+    "service_cmd_exec_failed" : "Unable to execute command '%s'",
+
+    "ldap_initialized" : "LDAP successfully initialized",
+    "password_too_short" : "Password is too short",
+    "admin_password_change_failed" : "Unable to change password",
+    "admin_password_changed" : "Administration password successfully changed",
+    "maindomain_change_failed" : "Unable to change main domain",
+    "maindomain_changed" : "Main domain successfully changed",
+    "yunohost_installing" : "Installing YunoHost...",
+    "yunohost_already_installed" : "YunoHost is already installed",
+    "yunohost_ca_creation_failed" : "Unable to create certificate authority",
+    "yunohost_configured" : "YunoHost successfully configured",
+    "update_cache_failed" : "Unable to update APT cache",
+    "system_no_upgrade" : "There is no packages to upgrade",
+    "system_upgraded" : "System successfully upgraded",
+
+    "field_invalid" : "Invalid field '%s'",
+    "mail_domain_unknown" : "Unknown mail address domain '%s'",
+    "mail_alias_remove_failed" : "Unable to remove mail alias '%s'",
+    "mail_forward_remove_failed" : "Unable to remove mail forward '%s'",
+    "user_unknown" : "Unknown user",
+    "user_creation_failed" : "Unable to create user",
+    "user_created" : "User successfully created",
+    "user_deletion_failed" : "Unable to delete user",
+    "user_deleted" : "User successfully deleted",
+    "user_update_failed" : "Unable to update user",
+    "user_updated" : "User successfully updated",
+    "user_info_failed" : "Unable to retrieve user information"
+}
+
diff --git a/lib/yunohost/monitor.py b/lib/yunohost/monitor.py
index d831befb..a2e7f8a1 100644
--- a/lib/yunohost/monitor.py
+++ b/lib/yunohost/monitor.py
@@ -31,6 +31,7 @@ import calendar
 import subprocess
 import xmlrpclib
 import os.path
+import errno
 import cPickle as pickle
 from urllib import urlopen
 from datetime import datetime, timedelta
@@ -74,7 +75,7 @@ def monitor_disk(units=None, mountpoint=None, human_readable=False):
             result_dname = dn
     if len(devices) == 0:
         if mountpoint is not None:
-            raise MoulinetteError(1, _("Unknown mountpoint '%s'") % mountpoint)
+            raise MoulinetteError(errno.ENODEV, m18n.n('mountpoint_unknown'))
         return result
 
     # Retrieve monitoring for unit(s)
@@ -131,7 +132,7 @@ def monitor_disk(units=None, mountpoint=None, human_readable=False):
             for dname in devices_names:
                 _set(dname, 'not-available')
         else:
-            raise MoulinetteError(1, _("Unknown unit '%s'") % u)
+            raise MoulinetteError(errno.EINVAL, m18n.n('unit_unknown') % u)
 
     if result_dname is not None:
         return result[result_dname]
@@ -203,7 +204,7 @@ def monitor_network(units=None, human_readable=False):
                 'gateway': gateway
             }
         else:
-            raise MoulinetteError(1, _("Unknown unit '%s'") % u)
+            raise MoulinetteError(errno.EINVAL, m18n.n('unit_unknown') % u)
 
     if len(units) == 1:
         return result[units[0]]
@@ -253,7 +254,7 @@ def monitor_system(units=None, human_readable=False):
         elif u == 'infos':
             result[u] = json.loads(glances.getSystem())
         else:
-            raise MoulinetteError(1, _("Unknown unit '%s'") % u)
+            raise MoulinetteError(errno.EINVAL, m18n.n('unit_unknown') % u)
 
     if len(units) == 1 and type(result[units[0]]) is not str:
         return result[units[0]]
@@ -269,7 +270,7 @@ def monitor_update_stats(period):
 
     """
     if period not in ['day', 'week', 'month']:
-        raise MoulinetteError(22, _("Invalid period"))
+        raise MoulinetteError(errno.EINVAL, m18n.n('monitor_period_invalid'))
 
     stats = _retrieve_stats(period)
     if not stats:
@@ -287,7 +288,7 @@ def monitor_update_stats(period):
         else:
             monitor = _monitor_all(p, 0)
     if not monitor:
-        raise MoulinetteError(1, _("No monitoring statistics to update"))
+        raise MoulinetteError(errno.ENODATA, m18n.n('monitor_stats_no_update'))
 
     stats['timestamp'].append(time.time())
 
@@ -352,13 +353,15 @@ def monitor_show_stats(period, date=None):
 
     """
     if period not in ['day', 'week', 'month']:
-        raise MoulinetteError(22, _("Invalid period"))
+        raise MoulinetteError(errno.EINVAL, m18n.n('monitor_period_invalid'))
 
     result = _retrieve_stats(period, date)
     if result is False:
-        raise MoulinetteError(167, _("Stats file not found"))
+        raise MoulinetteError(errno.ENOENT,
+                              m18n.n('monitor_stats_file_not_found'))
     elif result is None:
-        raise MoulinetteError(1, _("No available stats for the given period"))
+        raise MoulinetteError(errno.EINVAL,
+                              m18n.n('monitor_stats_period_unavailable'))
     return result
 
 
@@ -389,7 +392,7 @@ def monitor_enable(no_stats=False):
         os.system("touch %s" % crontab_path)
         os.system("echo '%s' >%s" % (rules, crontab_path))
 
-    msignals.display(_("Server monitoring successfully enabled."), 'success')
+    msignals.display(m18n.n('monitor_enabled'), 'success')
 
 
 def monitor_disable():
@@ -407,7 +410,7 @@ def monitor_disable():
         try:
             service_disable('glances')
         except MoulinetteError as e:
-            msignals.display('%s.' % e.strerror, 'warning')
+            msignals.display(e.strerror, 'warning')
 
     # Remove crontab
     try:
@@ -415,7 +418,7 @@ def monitor_disable():
     except:
         pass
 
-    msignals.display(_("Server monitoring successfully disabled."), 'success')
+    msignals.display(m18n.n('monitor_disabled'), 'success')
 
 
 def _get_glances_api():
@@ -434,8 +437,8 @@ def _get_glances_api():
     from yunohost.service import service_status
 
     if service_status('glances')['status'] != 'running':
-        raise MoulinetteError(1, _("Monitoring is disabled"))
-    raise MoulinetteError(1, _("Connection to Glances server failed"))
+        raise MoulinetteError(errno.EPERM, m18n.n('monitor_not_enabled'))
+    raise MoulinetteError(errno.EIO, m18n.n('monitor_glances_con_failed'))
 
 
 def _extract_inet(string, skip_netmask=False, skip_loopback=True):
diff --git a/lib/yunohost/service.py b/lib/yunohost/service.py
index ff796bb6..c2582c4d 100644
--- a/lib/yunohost/service.py
+++ b/lib/yunohost/service.py
@@ -26,6 +26,7 @@
 import yaml
 import glob
 import subprocess
+import errno
 import os.path
 
 from moulinette.core import MoulinetteError
@@ -43,11 +44,12 @@ def service_start(names):
         names = [names]
     for name in names:
         if _run_service_command('start', name):
-            msignals.display(_("Service '%s' successfully started.") % name, 'success')
+            msignals.display(m18n.n('service_started') % name, 'success')
         else:
             if service_status(name)['status'] != 'running':
-                raise MoulinetteError(1, _("Starting of service '%s' failed") % name)
-            msignals.display(_("Service '%s' already started.") % name)
+                raise MoulinetteError(errno.EPERM,
+                                      m18n.n('service_start_failed') % name)
+            msignals.display(m18n.n('service_already_started') % name)
 
 
 def service_stop(names):
@@ -62,11 +64,12 @@ def service_stop(names):
         names = [names]
     for name in names:
         if _run_service_command('stop', name):
-            msignals.display(_("Service '%s' successfully stopped.") % name, 'success')
+            msignals.display(m18n.n('service_stopped') % name, 'success')
         else:
             if service_status(name)['status'] != 'inactive':
-                raise MoulinetteError(1, _("Stopping of service '%s' failed") % name)
-            msignals.display(_("Service '%s' already stopped.") % name)
+                raise MoulinetteError(errno.EPERM,
+                                      m18n.n('service_stop_failed') % name)
+            msignals.display(m18n.n('service_already_stopped') % name)
 
 
 def service_enable(names):
@@ -81,9 +84,10 @@ def service_enable(names):
         names = [names]
     for name in names:
         if _run_service_command('enable', name):
-            msignals.display(_("Service '%s' successfully enabled.") % name, 'success')
+            msignals.display(m18n.n('service_enabled') % name, 'success')
         else:
-            raise MoulinetteError(1, _("Enabling of service '%s' failed") % name)
+            raise MoulinetteError(errno.EPERM,
+                                  m18n.n('service_enable_failed') % name)
 
 
 def service_disable(names):
@@ -98,9 +102,10 @@ def service_disable(names):
         names = [names]
     for name in names:
         if _run_service_command('disable', name):
-            msignals.display(_("Service '%s' successfully disabled.") % name, 'success')
+            msignals.display(m18n.n('service_disabled') % name, 'success')
         else:
-            raise MoulinetteError(1, _("Disabling of service '%s' failed") % name)
+            raise MoulinetteError(errno.EPERM,
+                                  m18n.n('service_disable_failed') % name)
 
 
 def service_status(names=[]):
@@ -123,7 +128,8 @@ def service_status(names=[]):
 
     for name in names:
         if check_names and name not in services.keys():
-            raise MoulinetteError(1, _("Unknown service '%s'") % name)
+            raise MoulinetteError(errno.EINVAL,
+                                  m18n.n('service_unknown') % name)
 
         status = None
         if services[name]['status'] == 'service':
@@ -145,8 +151,8 @@ def service_status(names=[]):
                 result[name]['status'] = 'inactive'
             else:
                 # TODO: Log output?
-                msignals.display(_("Could not determine status of service '%s'.") % \
-                                     name, 'warning')
+                msignals.display(m18n.n('service_status_failed') % name,
+                                 'warning')
         else:
             result[name]['status'] = 'running'
 
@@ -169,14 +175,14 @@ def service_log(name, number=50):
     Log every log files of a service
 
     Keyword argument:
-        name -- Services name to log
+        name -- Service name to log
         number -- Number of lines to display
 
     """
     services = _get_services()
 
     if name not in services.keys():
-        raise MoulinetteError(1, _("Unknown service '%s'") % service)
+        raise MoulinetteError(errno.EINVAL, m18n.n('service_unknown') % name)
 
     if 'log' in services[name]:
         log_list = services[name]['log']
@@ -191,7 +197,7 @@ def service_log(name, number=50):
             else:
                 result[log_path] = _tail(log_path, int(number))
     else:
-        raise MoulinetteError(1, _("Nothing to log for service '%s'") % name)
+        raise MoulinetteError(errno.EPERM, m18n.n('service_no_log') % name)
 
     return result
 
@@ -206,7 +212,7 @@ def _run_service_command(action, service):
 
     """
     if service not in _get_services().keys():
-        raise MoulinetteError(1, _("Unknown service '%s'") % service)
+        raise MoulinetteError(errno.EINVAL, m18n.n('service_unknown') % name)
 
     cmd = None
     if action in ['start', 'stop']:
@@ -215,14 +221,14 @@ def _run_service_command(action, service):
         arg = 'defaults' if action == 'enable' else 'remove'
         cmd = 'update-rc.d %s %s' % (service, arg)
     else:
-        raise MoulinetteError(1, _("Unknown action '%s'") % action)
+        raise ValueError("Unknown action '%s'" % action)
 
     try:
         ret = subprocess.check_output(cmd.split(), stderr=subprocess.STDOUT)
     except subprocess.CalledProcessError as e:
         # TODO: Log output?
-        msignals.display(_("Execution of command '%s' failed.") % \
-                             ' '.join(e.cmd), 'warning')
+        msignals.display(m18n.n('service_cmd_exec_failed')  % ' '.join(e.cmd),
+                         'warning')
         return False
     return True
 
diff --git a/lib/yunohost/tools.py b/lib/yunohost/tools.py
index 617ea8a0..5c634b54 100644
--- a/lib/yunohost/tools.py
+++ b/lib/yunohost/tools.py
@@ -30,6 +30,7 @@ import re
 import getpass
 import requests
 import json
+import errno
 import apt
 import apt.progress
 
@@ -69,7 +70,7 @@ def tools_ldapinit(auth):
 
     auth.update('cn=admin', admin_dict)
 
-    msignals.display(_("LDAP has been successfully initialized"), 'success')
+    msignals.display(m18n.n('ldap_ initialized'), 'success')
 
 
 def tools_adminpw(old_password, new_password):
@@ -83,7 +84,7 @@ def tools_adminpw(old_password, new_password):
     """
     # Validate password length
     if len(new_password) < 4:
-        raise MoulinetteError(22, _("Password is too short"))
+        raise MoulinetteError(errno.EINVAL, m18n.n('password_too_short'))
 
     old_password.replace('"', '\\"')
     old_password.replace('&', '\\&')
@@ -92,9 +93,10 @@ def tools_adminpw(old_password, new_password):
     result = os.system('ldappasswd -h localhost -D cn=admin,dc=yunohost,dc=org -w "%s" -a "%s" -s "%s"' % (old_password, old_password, new_password))
 
     if result == 0:
-        msignals.display(_("Admin password has been changed"), 'success')
+        msignals.display(m18n.n('admin_password_changed'), 'success')
     else:
-        raise MoulinetteError(22, _("Invalid password"))
+        raise MoulinetteError(errno.EPERM,
+                              m18n.n('admin_password_change_failed'))
 
 
 def tools_maindomain(auth, old_domain=None, new_domain=None, dyndns=False):
@@ -165,7 +167,8 @@ def tools_maindomain(auth, old_domain=None, new_domain=None, dyndns=False):
 
     for command in command_list:
         if os.system(command) != 0:
-            raise MoulinetteError(17, _("There were a problem during domain changing"))
+            raise MoulinetteError(errno.EPERM,
+                                  m18n.n('maindomain_change_failed'))
 
     if dyndns: dyndns_subscribe(domain=new_domain)
     elif len(new_domain.split('.')) >= 3:
@@ -175,7 +178,7 @@ def tools_maindomain(auth, old_domain=None, new_domain=None, dyndns=False):
         if dyndomain in dyndomains:
             dyndns_subscribe(domain=new_domain)
 
-    msignals.display(_("Main domain has been successfully changed"), 'success')
+    msignals.display(m18n.n('maindomain_changed'), 'success')
 
 
 def tools_postinstall(domain, password, dyndns=False):
@@ -194,9 +197,9 @@ def tools_postinstall(domain, password, dyndns=False):
     try:
         with open('/etc/yunohost/installed') as f: pass
     except IOError:
-        print('Installing YunoHost')
+        msignals.display(m18n.n('yunohost_installing'))
     else:
-        raise MoulinetteError(17, _("YunoHost is already installed"))
+        raise MoulinetteError(errno.EPERM, m18n.n('yunohost_already_installed'))
 
     if len(domain.split('.')) >= 3:
         r = requests.get('http://dyndns.yunohost.org/domains')
@@ -206,7 +209,8 @@ def tools_postinstall(domain, password, dyndns=False):
             if requests.get('http://dyndns.yunohost.org/test/%s' % domain).status_code == 200:
                 dyndns=True
             else:
-                raise MoulinetteError(17, _("Domain is already taken"))
+                raise MoulinetteError(errno.EEXIST,
+                                      m18n.n('dyndns_unavailable'))
 
     # Create required folders
     folders_to_create = [
@@ -257,7 +261,8 @@ def tools_postinstall(domain, password, dyndns=False):
 
     for command in command_list:
         if os.system(command) != 0:
-            raise MoulinetteError(17, _("There were a problem during CA creation"))
+            raise MoulinetteError(errno.EPERM,
+                                  m18n.n('yunohost_ca_creation_failed'))
 
     # Initialize YunoHost LDAP base
     tools_ldapinit(auth)
@@ -277,7 +282,7 @@ def tools_postinstall(domain, password, dyndns=False):
     os.system('touch /etc/yunohost/installed')
     os.system('service yunohost-api restart &')
 
-    msignals.display(_("YunoHost has been successfully configured"), 'success')
+    msignals.display(m18n.n('yunohost_configured'), 'success')
 
 
 def tools_update(ignore_apps=False, ignore_packages=False):
@@ -296,7 +301,7 @@ def tools_update(ignore_apps=False, ignore_packages=False):
         cache = apt.Cache()
         # Update APT cache
         if not cache.update():
-            raise MoulinetteError(1, _("An error occured during APT cache update"))
+            raise MoulinetteError(errno.EPERM, m18n.n('update_cache_failed'))
 
         cache.open(None)
         cache.upgrade(True)
@@ -338,12 +343,11 @@ def tools_update(ignore_apps=False, ignore_packages=False):
                     })
     
     if len(apps) == 0 and len(packages) == 0:
-        msignals.display(_("There is nothing to upgrade right now"), 'success')
+        msignals.display(m18n.n('system_no_upgrade'), 'success')
 
     return { 'packages': packages, 'apps': apps }
 
 
-
 def tools_upgrade(ignore_apps=False, ignore_packages=False):
     """
     Update apps & package cache, then display changelog
@@ -380,7 +384,7 @@ def tools_upgrade(ignore_apps=False, ignore_packages=False):
             app_upgrade()
         except: pass
 
-    msignals.display(_("System successfully upgraded"), 'success')
+    msignals.display(m18n.n('system_upgraded'), 'success')
 
     # Return API logs if it is an API call
     if not os.isatty(1):
diff --git a/lib/yunohost/user.py b/lib/yunohost/user.py
index 87b1482d..71a76f9e 100644
--- a/lib/yunohost/user.py
+++ b/lib/yunohost/user.py
@@ -29,6 +29,7 @@ import crypt
 import random
 import string
 import json
+import errno
 
 from moulinette.core import MoulinetteError
 
@@ -64,7 +65,8 @@ def user_list(auth, fields=None, filter=None, limit=None, offset=None):
             if attr in keys:
                 attrs.append(attr)
             else:
-                raise MoulinetteError(22, _("Invalid field '%s'") % attr)
+                raise MoulinetteError(errno.EINVAL,
+                                      m18n.n('field_invalid') % attr)
     else:
         attrs = [ 'uid', 'cn', 'mail' ]
 
@@ -99,7 +101,7 @@ def user_create(auth, username, firstname, lastname, mail, password):
 
     # Validate password length
     if len(password) < 4:
-        raise MoulinetteError(22, _("Password is too short"))
+        raise MoulinetteError(errno.EINVAL, m18n.n('password_too_short'))
 
     auth.validate_uniqueness({
         'uid'       : username,
@@ -107,10 +109,11 @@ def user_create(auth, username, firstname, lastname, mail, password):
     })
 
     if mail[mail.find('@')+1:] not in domain_list(auth)['domains']:
-        raise MoulinetteError(22, _("Unknown domain '%s'") % mail[mail.find('@')+1:])
+        raise MoulinetteError(errno.EINVAL,
+                              m18n.n('mail_domain_unknown')
+                                      % mail[mail.find('@')+1:])
 
     # Get random UID/GID
-
     uid_check = gid_check = 0
     while uid_check == 0 and gid_check == 0:
         uid = str(random.randint(200, 99999))
@@ -174,12 +177,12 @@ def user_create(auth, username, firstname, lastname, mail, password):
             os.system("su - %s -c ''" % username)
             os.system('yunohost app ssowatconf > /dev/null 2>&1')
             #TODO: Send a welcome mail to user
-            msignals.display(_("User '%s' successfully created.") % username, 'success')
+            msignals.display(m18n.n('user_created'), 'success')
             hook_callback('post_user_create', [username, mail, password, firstname, lastname])
 
             return { 'fullname' : fullname, 'username' : username, 'mail' : mail }
 
-    raise MoulinetteError(169, _("An error occurred during user creation"))
+    raise MoulinetteError(169, m18n.n('user_creation_failed'))
 
 
 def user_delete(auth, users, purge=False):
@@ -207,10 +210,10 @@ def user_delete(auth, users, purge=False):
                 deleted.append(user)
                 continue
         else:
-            raise MoulinetteError(169, _("An error occurred during user deletion"))
+            raise MoulinetteError(169, m18n.n('user_deletion_failed'))
 
     os.system('yunohost app ssowatconf > /dev/null 2>&1')
-    msignals.display(_("User(s) successfully deleted."), 'success')
+    msignals.display(m18n.n('user_deleted'), 'success')
     return { 'users': deleted }
 
 
@@ -239,7 +242,7 @@ def user_update(auth, username, firstname=None, lastname=None, mail=None, change
     # Populate user informations
     result = auth.search(base='ou=users,dc=yunohost,dc=org', filter='uid=' + username, attrs=attrs_to_fetch)
     if not result:
-        raise MoulinetteError(167, _("Unknown username '%s'") % username)
+        raise MoulinetteError(errno.EINVAL, m18n.n('user_unknown'))
     user = result[0]
 
     # Get modifications from arguments
@@ -263,7 +266,9 @@ def user_update(auth, username, firstname=None, lastname=None, mail=None, change
     if mail:
         auth.validate_uniqueness({ 'mail': mail })
         if mail[mail.find('@')+1:] not in domains:
-            raise MoulinetteError(22, _("Unknown domain '%s'") % mail[mail.find('@')+1:])
+            raise MoulinetteError(errno.EINVAL,
+                                  m18n.n('mail_domain_unknown')
+                                          % mail[mail.find('@')+1:])
         del user['mail'][0]
         new_attr_dict['mail'] = [mail] + user['mail']
 
@@ -273,7 +278,9 @@ def user_update(auth, username, firstname=None, lastname=None, mail=None, change
         for mail in add_mailalias:
             auth.validate_uniqueness({ 'mail': mail })
             if mail[mail.find('@')+1:] not in domains:
-                raise MoulinetteError(22, _("Unknown domain '%s'") % mail[mail.find('@')+1:])
+                raise MoulinetteError(errno.EINVAL,
+                                      m18n.n('mail_domain_unknown')
+                                              % mail[mail.find('@')+1:])
             user['mail'].append(mail)
         new_attr_dict['mail'] = user['mail']
 
@@ -284,7 +291,8 @@ def user_update(auth, username, firstname=None, lastname=None, mail=None, change
             if len(user['mail']) > 1 and mail in user['mail'][1:]:
                 user['mail'].remove(mail)
             else:
-                raise MoulinetteError(22, _("Invalid mail alias '%s'") % mail)
+                raise MoulinetteError(errno.EINVAL,
+                                      m18n.n('mail_alias_remove_failed') % mail)
         new_attr_dict['mail'] = user['mail']
 
     if add_mailforward:
@@ -303,14 +311,15 @@ def user_update(auth, username, firstname=None, lastname=None, mail=None, change
             if len(user['maildrop']) > 1 and mail in user['maildrop'][1:]:
                 user['maildrop'].remove(mail)
             else:
-                raise MoulinetteError(22, _("Invalid mail forward '%s'") % mail)
+                raise MoulinetteError(errno.EINVAL,
+                                      m18n.n('mail_forward_remove_failed') % mail)
         new_attr_dict['maildrop'] = user['maildrop']
 
     if auth.update('uid=%s,ou=users' % username, new_attr_dict):
-       msignals.display(_("User '%s' successfully updated.") % username, 'success')
+       msignals.display(m18n.n('user_updated'), 'success')
        return user_info(auth, username)
     else:
-       raise MoulinetteError(169, _("An error occurred during user update"))
+       raise MoulinetteError(169, m18n.n('user_update_failed'))
 
 
 def user_info(auth, username):
@@ -333,7 +342,7 @@ def user_info(auth, username):
     if result:
         user = result[0]
     else:
-        raise MoulinetteError(22, _("Unknown username/mail '%s'") % username)
+        raise MoulinetteError(errno.EINVAL, m18n.n('user_unknown'))
 
     result_dict = {
         'username': user['uid'][0],
@@ -352,4 +361,4 @@ def user_info(auth, username):
     if result:
         return result_dict
     else:
-        raise MoulinetteError(167, _("No user found"))
+        raise MoulinetteError(167, m18n.n('user_info_failed'))
diff --git a/locales/en.json b/locales/en.json
new file mode 100644
index 00000000..8a75fec5
--- /dev/null
+++ b/locales/en.json
@@ -0,0 +1,26 @@
+{
+    "colon" : "%s: ",
+
+    "success" : "Success!",
+    "warning" : "Warning:",
+    "error" : "Error:",
+    "permission_denied" : "Permission denied",
+    "root_required" : "You must be root to perform this action",
+    "instance_already_running" : "An instance is already running",
+
+    "unable_authenticate" : "Unable to authenticate",
+    "unable_retrieve_session" : "Unable to retrieve the session",
+    "error_ldap_operation" : "An error occured during LDAP operation",
+
+    "password" : "Password",
+    "invalid_password" : "Invalid password",
+    "confirm" : "Confirm",
+    "values_mismatch" : "Values don't match",
+    "authentication_required_long" : "Authentication is required to perform this action",
+    "authentication_required" : "Authentication required",
+    "authentication_profile_required" : "Authentication to profile '%s' required",
+    "operation_interrupted" : "Operation interrupted",
+
+    "not_logged_in" : "You are not logged in",
+    "server_already_running" : "A server is already running on that port"
+}
diff --git a/locales/fr.json b/locales/fr.json
new file mode 100644
index 00000000..90a321ae
--- /dev/null
+++ b/locales/fr.json
@@ -0,0 +1,26 @@
+{
+    "colon" : "%s : ",
+
+    "success" : "Succès !",
+    "warning" : "Attention :",
+    "error" : "Erreur :",
+    "permission_denied" : "Permission refusée",
+    "root_required" : "Vous devez avoir les droits super-utilisateur pour exécuter cette action",
+    "instance_already_running" : "Une instance est déjà en cours d'exécution",
+
+    "unable_authenticate" : "Impossible de vous authentifier",
+    "unable_retrieve_session" : "Impossible de récupérer la session",
+    "error_ldap_operation" : "Une erreur est survenue lors de l'opération LDAP",
+
+    "password" : "Mot de passe",
+    "invalid_password" : "Mot de passe incorrect",
+    "confirm" : "Confirmez",
+    "values_mismatch" : "Les valeurs ne correspondent pas",
+    "authentication_required_long" : "L'authentification est requise pour exécuter cette action",
+    "authentication_required" : "Authentification requise",
+    "authentication_profile_required" : "Authentification au profile '%s' requise",
+    "operation_interrupted" : "Opération interrompue",
+
+    "not_logged_in" : "Vous n'êtes pas connecté",
+    "server_already_running" : "Un server est déjà en cours d'exécution sur ce port"
+}
diff --git a/moulinette/__init__.py b/moulinette/__init__.py
index 5c53cd6a..066a1f0b 100755
--- a/moulinette/__init__.py
+++ b/moulinette/__init__.py
@@ -50,13 +50,11 @@ def init(**kwargs):
     """
     import sys
     import __builtin__
-    from moulinette.core import Package, MoulinetteSignals, install_i18n
+    from moulinette.core import Package, Moulinette18n, MoulinetteSignals
     __builtin__.__dict__['pkg'] = Package(**kwargs)
+    __builtin__.__dict__['m18n'] = Moulinette18n(pkg)
     __builtin__.__dict__['msignals'] = MoulinetteSignals()
 
-    # Initialize internationalization
-    install_i18n()
-
     # Add library directory to python path
     sys.path.insert(0, pkg.libdir)
 
@@ -104,6 +102,6 @@ def cli(namespaces, args, use_cache=True):
                                                 'use_cache': use_cache})
         moulinette.run(args)
     except MoulinetteError as e:
-        print(_('%s: %s' % (colorize(_('Error'), 'red'), e.strerror)))
+        print('%s %s' % (colorize(m18n.g('error'), 'red'), e.strerror))
         return e.errno
     return 0
diff --git a/moulinette/actionsmap.py b/moulinette/actionsmap.py
index 880ee420..5c35ceda 100644
--- a/moulinette/actionsmap.py
+++ b/moulinette/actionsmap.py
@@ -259,7 +259,7 @@ class ActionsMap(object):
     def __init__(self, parser, namespaces=[], use_cache=True):
         self.use_cache = use_cache
         if not issubclass(parser, BaseActionsMapParser):
-            raise MoulinetteError(errno.EINVAL, _("Invalid parser class '%s'" % parser.__name__))
+            raise ValueError("Invalid parser class '%s'" % parser.__name__)
         self._parser_class = parser
 
         logging.debug("initializing ActionsMap for the interface '%s'" % parser.interface)
@@ -311,7 +311,7 @@ class ActionsMap(object):
         try:
             auth = self.parser.get_global_conf('authenticator', profile)[1]
         except KeyError:
-            raise MoulinetteError(errno.EINVAL, _("Unknown authenticator profile '%s'") % profile)
+            raise ValueError("Unknown authenticator profile '%s'" % profile)
         else:
             return auth()
 
@@ -343,9 +343,11 @@ class ActionsMap(object):
                                  fromlist=[func_name])
                 func = getattr(mod, func_name)
             except (AttributeError, ImportError):
-                raise MoulinetteError(errno.ENOSYS, _('Function is not defined'))
+                raise ImportError("Unable to load function %s.%s/%s"
+                        % (namespace, category, func_name))
             else:
-                # Process the action
+                # Load translation and process the action
+                m18n.load_namespace(namespace)
                 return func(**arguments)
 
     @staticmethod
diff --git a/moulinette/authenticators/__init__.py b/moulinette/authenticators/__init__.py
index 601f7d06..b9c33134 100644
--- a/moulinette/authenticators/__init__.py
+++ b/moulinette/authenticators/__init__.py
@@ -93,7 +93,7 @@ class BaseAuthenticator(object):
                 s_id, s_hash = token
             except TypeError:
                 if not password:
-                    raise MoulinetteError(errno.EINVAL, _("Invalid format for token"))
+                    raise ValueError("Invalid token format")
                 else:
                     # TODO: Log error
                     store_session = False
@@ -108,9 +108,9 @@ class BaseAuthenticator(object):
         except MoulinetteError:
             raise
         except Exception as e:
-            logging.error("authentication (name: '%s', type: '%s') fails: %s" % \
-                              (self.name, self.vendor, e))
-            raise MoulinetteError(errno.EACCES, _("Unable to authenticate"))
+            logging.error("authentication (name: '%s', type: '%s') fails: %s" \
+                    % (self.name, self.vendor, e))
+            raise MoulinetteError(errno.EACCES, m18n.g('unable_authenticate'))
 
         # Store session
         if store_session:
@@ -140,8 +140,8 @@ class BaseAuthenticator(object):
             with self._open_sessionfile(session_id, 'r') as f:
                 enc_pwd = f.read()
         except IOError:
-            # TODO: Set proper error code
-            raise MoulinetteError(167, _("Unable to retrieve session"))
+            raise MoulinetteError(errno.ENOENT,
+                                  m18r.g('unable_retrieve_session'))
         else:
             gpg = gnupg.GPG()
             gpg.encoding = 'utf-8'
diff --git a/moulinette/authenticators/ldap.py b/moulinette/authenticators/ldap.py
index 9e81acf7..1855a79f 100644
--- a/moulinette/authenticators/ldap.py
+++ b/moulinette/authenticators/ldap.py
@@ -2,6 +2,7 @@
 
 # TODO: Use Python3 to remove this fix!
 from __future__ import absolute_import
+import errno
 import ldap
 import ldap.modlist as modlist
 
@@ -64,7 +65,7 @@ class Authenticator(BaseAuthenticator):
             else:
                 con.simple_bind_s()
         except ldap.INVALID_CREDENTIALS:
-            raise MoulinetteError(errno.EACCES, _("Invalid password"))
+            raise MoulinetteError(errno.EACCES, m18n.g('invalid_password'))
         else:
             self.con = con
 
@@ -93,7 +94,7 @@ class Authenticator(BaseAuthenticator):
         try:
             result = self.con.search_s(base, ldap.SCOPE_SUBTREE, filter, attrs)
         except:
-            raise MoulinetteError(169, _('An error occured during LDAP search'))
+            raise MoulinetteError(169, m18n.g('error_ldap_operation'))
 
         result_list = []
         if not attrs or 'dn' not in attrs:
@@ -122,7 +123,7 @@ class Authenticator(BaseAuthenticator):
         try:
             self.con.add_s(dn, ldif)
         except:
-            raise MoulinetteError(169, _('An error occured during LDAP entry creation'))
+            raise MoulinetteError(169, m18n.g('error_ldap_operation'))
         else:
             return True
 
@@ -141,7 +142,7 @@ class Authenticator(BaseAuthenticator):
         try:
             self.con.delete_s(dn)
         except:
-            raise MoulinetteError(169, _('An error occured during LDAP entry deletion'))
+            raise MoulinetteError(169, m18n.g('error_ldap_operation'))
         else:
             return True
 
@@ -169,7 +170,7 @@ class Authenticator(BaseAuthenticator):
 
             self.con.modify_ext_s(dn, ldif)
         except:
-            raise MoulinetteError(169, _('An error occured during LDAP entry update'))
+            raise MoulinetteError(169, m18n.g('error_ldap_operation'))
         else:
             return True
 
@@ -188,5 +189,5 @@ class Authenticator(BaseAuthenticator):
             if not self.search(filter=attr + '=' + value):
                 continue
             else:
-                raise MoulinetteError(17, _('Attribute already exists') + ' "' + attr + '=' + value + '"')
+                raise MoulinetteError(17, 'Attribute already exists "%s=%s"' % (attr, value))
         return True
diff --git a/moulinette/core.py b/moulinette/core.py
index 6ced832a..30de4efd 100644
--- a/moulinette/core.py
+++ b/moulinette/core.py
@@ -3,35 +3,14 @@
 import os
 import sys
 import time
+import json
 import errno
-import gettext
 import logging
 
 from importlib import import_module
 
 # Package manipulation -------------------------------------------------
 
-def install_i18n(namespace=None):
-    """Install internationalization
-
-    Install translation based on the package's default gettext domain or
-    on 'namespace' if provided.
-
-    Keyword arguments:
-        - namespace -- The namespace to initialize i18n for
-
-    """
-    if namespace:
-        try:
-            t = gettext.translation(namespace, pkg.localedir)
-        except IOError:
-            # TODO: Log error
-            return
-        else:
-            t.install()
-    else:
-        gettext.install('moulinette', pkg.localedir)
-
 class Package(object):
     """Package representation and easy access methods
 
@@ -51,7 +30,7 @@ class Package(object):
             # Set local directories
             self._datadir = '%s/data' % basedir
             self._libdir = '%s/lib' % basedir
-            self._localedir = '%s/po' % basedir
+            self._localedir = '%s/locales' % basedir
             self._cachedir = '%s/cache' % basedir
         else:
             import package
@@ -130,6 +109,189 @@ class Package(object):
                                         True if mode[0] == 'w' else False)
         return open('%s/%s' % (self.get_cachedir(**kwargs), filename), mode)
 
+
+# Internationalization -------------------------------------------------
+
+class Translator(object):
+    """Internationalization class
+
+    Provide an internationalization mechanism based on JSON files to
+    translate a key in the proper locale.
+
+    Keyword arguments:
+        - locale_dir -- The directory where locale files are located
+        - default_locale -- The default locale to use
+
+    """
+    def __init__(self, locale_dir, default_locale='en'):
+        self.locale_dir = locale_dir
+        self.locale = default_locale
+        self._translations = {}
+
+        # Attempt to load default translations
+        if not self._load_translations(default_locale):
+            raise ValueError("Unable to load locale '%s' from '%s'"
+                    % (default_locale, locale_dir))
+        self.default_locale = default_locale
+
+    def get_locales(self):
+        """Return a list of the avalaible locales"""
+        locales = []
+
+        for f in os.listdir(self.locale_dir):
+            if f.endswith('.json'):
+                # TODO: Validate locale
+                locales.append(f[:-5])
+        return locales
+
+    def set_locale(self, locale):
+        """Set the locale to use
+
+        Set the locale to use at first. If the locale is not available,
+        the default locale is used.
+
+        Keyword arguments:
+            - locale -- The locale to use
+
+        Returns:
+            True if the locale has been set, otherwise False
+
+        """
+        if locale not in self._translations:
+            if not self._load_translations(locale):
+                logging.info("unable to load locale '%s' from '%s'"
+                        % (self.default_locale, self.locale_dir))
+
+                # Revert to default locale
+                self.locale = self.default_locale
+                return False
+
+        # Set current locale
+        self.locale = locale
+        return True
+
+    def translate(self, key):
+        """Retrieve proper translation for a key
+
+        Attempt to retrieve translation for a key using the current locale
+        or the default locale if 'key' is not found.
+
+        Keyword arguments:
+            - key -- The key to translate
+
+        """
+        try:
+            value = self._translations[self.locale][key]
+        except KeyError:
+            try:
+                value = self._translations[self.default_locale][key]
+                logging.info("untranslated key '%s' for locale '%s'" %
+                        (key, self.locale))
+            except KeyError:
+                logging.warning("unknown key '%s' for locale '%s'" %
+                        (key, self.default_locale))
+                return key
+        return value
+
+    def _load_translations(self, locale, overwrite=False):
+        """Load translations for a locale
+
+        Attempt to load translations for a given locale. If 'overwrite' is
+        True, translations will be loaded again.
+
+        Keyword arguments:
+            - locale -- The locale to load
+            - overwrite -- True to overwrite existing translations
+
+        Returns:
+            True if the translations have been loaded, otherwise False
+
+        """
+        if not overwrite and locale in self._translations:
+            return True
+
+        try:
+            with open('%s/%s.json' % (self.locale_dir, locale), 'r') as f:
+                j = json.load(f)
+        except IOError:
+            return False
+        else:
+            self._translations[locale] = j
+        return True
+
+
+class Moulinette18n(object):
+    """Internationalization service for the moulinette
+
+    Manage internationalization and access to the proper keys translation
+    used in the moulinette and libraries.
+
+    Keyword arguments:
+        - package -- The current Package instance
+        - default_locale -- The default locale to use
+
+    """
+    def __init__(self, package, default_locale='en'):
+        self.default_locale = default_locale
+        self.locale = default_locale
+        self.pkg = package
+
+        # Init translators
+        self._global = Translator(self.pkg.localedir, default_locale)
+        self._namespace = None
+
+    def load_namespace(self, namespace):
+        """Load the namespace to use
+
+        Load and set translations of a given namespace. Those translations
+        are accessible with Moulinette18n.n().
+
+        Keyword arguments:
+            - namespace -- The namespace to load
+
+        """
+        if self._namespace and self._namespace[0] == namespace:
+            return
+
+        self._namespace = (namespace, Translator('%s/%s/locales'
+                % (self.pkg.libdir, namespace), self.default_locale))
+        self._namespace[1].set_locale(self.locale)
+
+    def set_locale(self, locale):
+        """Set the locale to use"""
+        self.locale = locale
+
+        self._global.set_locale(locale)
+        if self._namespace:
+            self._namespace[1].set_locale(locale)
+
+    def g(self, key):
+        """Retrieve proper translation for a moulinette key
+
+        Attempt to retrieve value for a key from moulinette translations
+        using the current locale or the default locale if 'key' is not found.
+
+        Keyword arguments:
+            - key -- The key to translate
+
+        """
+        return self._global.translate(key)
+
+    def n(self, key):
+        """Retrieve proper translation for a moulinette key
+
+        Attempt to retrieve value for a key from loaded namespace translations
+        using the current locale or the default locale if 'key' is not found.
+
+        Keyword arguments:
+            - key -- The key to translate
+
+        """
+        if not self._namespace:
+            raise RuntimeError("No namespace loaded for translation")
+        return self._namespace[1].translate(key)
+
+
 class MoulinetteSignals(object):
     """Signals connector for the moulinette
 
@@ -256,16 +418,16 @@ def init_interface(name, kwargs={}, actionsmap={}):
 
     try:
         mod = import_module('moulinette.interfaces.%s' % name)
-    except ImportError:
+    except ImportError as e:
         # TODO: List available interfaces
-        raise MoulinetteError(errno.EINVAL, _("Unknown interface '%s'" % name))
+        raise ImportError("Unable to load interface '%s': %s" % (name, str(e)))
     else:
         try:
             # Retrieve interface classes
             parser = mod.ActionsMapParser
             interface = mod.Interface
         except AttributeError as e:
-            raise MoulinetteError(errno.EFAULT, _("Invalid interface '%s': %s") % (name, e))
+            raise ImportError("Invalid interface '%s': %s" % (name, e))
 
     # Instantiate or retrieve ActionsMap
     if isinstance(actionsmap, dict):
@@ -273,7 +435,7 @@ def init_interface(name, kwargs={}, actionsmap={}):
     elif isinstance(actionsmap, ActionsMap):
         amap = actionsmap
     else:
-        raise MoulinetteError(errno.EINVAL, _("Invalid actions map '%r'" % actionsmap))
+        raise ValueError("Invalid actions map '%r'" % actionsmap)
 
     return interface(amap, **kwargs)
 
@@ -293,7 +455,8 @@ def init_authenticator((vendor, name), kwargs={}):
         mod = import_module('moulinette.authenticators.%s' % vendor)
     except ImportError as e:
         # TODO: List available authenticators vendors
-        raise MoulinetteError(errno.EINVAL, _("Unable to load authenticator vendor '%s': %s") % (vendor, str(e)))
+        raise ImportError("Unable to load authenticator vendor '%s': %s"
+                % (vendor, str(e)))
     else:
         return mod.Authenticator(name, **kwargs)
 
@@ -369,12 +532,13 @@ class MoulinetteLock(object):
                 try:
                     (open(self._lockfile, 'w')).close()
                 except IOError:
-                    raise MoulinetteError(errno.EPERM, _("Permission denied, did you forget 'sudo' ?"))
+                    raise MoulinetteError(errno.EPERM,
+                                          '%s. %s.' % (m18n.g('permission_denied'), m18n.g('root_required')))
                 break
 
             if (time.time() - start_time) > self.timeout:
-                raise MoulinetteError(errno.EBUSY, _("An instance is already running for '%s'") \
-                                          % self.namespace)
+                raise MoulinetteError(errno.EBUSY,
+                                      m18n.g('instance_already_running'))
             # Wait before checking again
             time.sleep(self.interval)
         self._locked = True
diff --git a/moulinette/interfaces/api.py b/moulinette/interfaces/api.py
index 22dbca93..be54ac3f 100644
--- a/moulinette/interfaces/api.py
+++ b/moulinette/interfaces/api.py
@@ -122,7 +122,7 @@ class _ActionsMapPlugin(object):
                 try:
                     kwargs['password'] = request.POST['password']
                 except KeyError:
-                    raise HTTPBadRequestResponse(_("Missing password parameter"))
+                    raise HTTPBadRequestResponse("Missing password parameter")
                 try:
                     kwargs['profile'] = request.POST['profile']
                 except KeyError:
@@ -235,7 +235,7 @@ class _ActionsMapPlugin(object):
         try:
             del self.secrets[s_id]
         except KeyError:
-            raise HTTPUnauthorizedResponse(_("You are not logged in"))
+            raise HTTPUnauthorizedResponse(m18n.g('not_logged_in'))
         else:
             # TODO: Clean the session for profile only
             # Delete cookie and clean the session
@@ -278,9 +278,9 @@ class _ActionsMapPlugin(object):
                                         secret=s_secret)[authenticator.name]
         except KeyError:
             if authenticator.name == 'default':
-                msg = _("Needing authentication")
+                msg = m18n.g('authentication_required')
             else:
-                msg = _("Needing authentication to profile '%s'") % authenticator.name
+                msg = m18n.g('authentication_profile_required') % authenticator.name
             raise HTTPUnauthorizedResponse(msg)
         else:
             return authenticator(token=(s_id, s_hash))
@@ -405,7 +405,7 @@ class ActionsMapParser(BaseActionsMapParser):
             auth = msignals.authenticate(klass(), **auth_conf)
             if not auth.is_authenticated:
                 # TODO: Set proper error code
-                raise MoulinetteError(errno.EACCES, _("This action need authentication"))
+                raise MoulinetteError(errno.EACCES, m18n.g('authentication_required_long'))
             if self.get_conf(tid, 'argument_auth') and \
                self.get_conf(tid, 'authenticate') == 'all':
                 ret.auth = auth
@@ -466,7 +466,8 @@ class Interface(BaseInterface):
             run(self._app, port=_port)
         except IOError as e:
             if e.args[0] == errno.EADDRINUSE:
-                raise MoulinetteError(errno.EADDRINUSE, _("A server is already running"))
+                raise MoulinetteError(errno.EADDRINUSE,
+                                      m18n.g('server_already_running'))
             raise
 
 
diff --git a/moulinette/interfaces/cli.py b/moulinette/interfaces/cli.py
index dc273357..c5b6b3c6 100644
--- a/moulinette/interfaces/cli.py
+++ b/moulinette/interfaces/cli.py
@@ -137,8 +137,8 @@ class ActionsMapParser(BaseActionsMapParser):
             # TODO: Catch errors
             auth = msignals.authenticate(klass(), **auth_conf)
             if not auth.is_authenticated:
-                # TODO: Set proper error code
-                raise MoulinetteError(errno.EACCES, _("This action need authentication"))
+                raise MoulinetteError(errno.EACCES,
+                                      m18n.g('authentication_required_long'))
             if self.get_conf(ret._tid, 'argument_auth') and \
                self.get_conf(ret._tid, 'authenticate') == 'all':
                 ret.auth = auth
@@ -157,6 +157,12 @@ class Interface(BaseInterface):
 
     """
     def __init__(self, actionsmap):
+        import locale
+
+        # Set user locale
+        lang = locale.getdefaultlocale()[0]
+        m18n.set_locale(lang[:2])
+
         # Connect signals to handlers
         msignals.set_handler('authenticate', self._do_authenticate)
         msignals.set_handler('display', self._do_display)
@@ -177,7 +183,7 @@ class Interface(BaseInterface):
         try:
             ret = self.actionsmap.process(args, timeout=5)
         except KeyboardInterrupt, EOFError:
-            raise MoulinetteError(errno.EINTR, _("Interrupted"))
+            raise MoulinetteError(errno.EINTR, m18n.g('operation_interrupted'))
 
         if isinstance(ret, dict):
             pretty_print_dict(ret)
@@ -194,7 +200,7 @@ class Interface(BaseInterface):
 
         """
         # TODO: Allow token authentication?
-        msg = help or _("Password")
+        msg = help or m18n.g('password')
         return authenticator(password=self._do_prompt(msg, True, False))
 
     def _do_prompt(self, message, is_password, confirm):
@@ -204,14 +210,15 @@ class Interface(BaseInterface):
 
         """
         if is_password:
-            prompt = lambda m: getpass.getpass(colorize(_('%s: ') % m, 'blue'))
+            prompt = lambda m: getpass.getpass(colorize(m18n.g('colon') % m,
+                                                        'blue'))
         else:
-            prompt = lambda m: raw_input(colorize(_('%s: ') % m, 'blue'))
+            prompt = lambda m: raw_input(colorize(m18n.g('colon') % m, 'blue'))
         value = prompt(message)
 
         if confirm:
-            if prompt(_('Retype %s') % message) != value:
-                raise MoulinetteError(errno.EINVAL, _("Values don't match"))
+            if prompt('%s %s' % (m18n.g('confirm'), message)) != value:
+                raise MoulinetteError(errno.EINVAL, m18n.g('values_mismatch'))
 
         return value
 
@@ -222,8 +229,8 @@ class Interface(BaseInterface):
 
         """
         if style == 'success':
-            print('%s %s' % (colorize(_("Success!"), 'green'), message))
+            print('%s %s' % (colorize(m18n.g('success'), 'green'), message))
         elif style == 'warning':
-            print('%s %s' % (colorize(_("Warning!"), 'yellow'), message))
+            print('%s %s' % (colorize(m18n.g('warning'), 'yellow'), message))
         else:
             print(message)