config: Don't restrict supported languages

- There is no reason to limit the number of languages supported by the
  application.  This is what happens when LANGUAGES is set during Django
  configuration.  Users should be able to set a language using browser
  preferences and see at least the Django translated messages even if
  Plinth messages are not translated (example user create/edit form).

- If we wish to limit the number of language options shown duing
  language selection (for quality perception reasons), we should perform
  the limiting at the time of configuration.

- Note that when a new language is requested in translation tool, we
  create an empty file and that may lead to language being shown but
  with few strings actually translated.  This will have poor impact on
  user experience.  I wonder if we should filter language at all,
  keeping this in mind.

plinth/__main__.py

@@ -21,6 +21,7 @@ import django.conf
 from django.contrib.messages import constants as message_constants
 import django.core.management
 import django.core.wsgi
+from django.utils import translation
 import importlib
 import logging
 import os
@@ -224,18 +225,6 @@ def configure_django():
     if cfg.secure_proxy_ssl_header:
         secure_proxy_ssl_header = (cfg.secure_proxy_ssl_header, 'https')
 
-    # Read translated languages from the 'locale' directory
-    languages = [('en', 'English')]
-    locale_dir = os.path.join(os.path.dirname(__file__), 'locale')
-    if os.path.isdir(locale_dir):
-        translated_language_codes = next(os.walk(locale_dir))[1]
-        all_languages = dict(django.conf.global_settings.LANGUAGES)
-        for code in translated_language_codes:
-            lang_code = code.replace('_', '-')
-            if lang_code in all_languages:
-                languages.append((code, all_languages[lang_code]))
-        languages = sorted(languages, key=lambda tup: tup[1])
-
     django.conf.settings.configure(
         ALLOWED_HOSTS=['*'],
         CACHES={'default':
@@ -245,7 +234,6 @@ def configure_django():
                     'NAME': cfg.store_file}},
         DEBUG=cfg.debug,
         INSTALLED_APPS=applications,
-        LANGUAGES=languages,
         LOGGING=logging_configuration,
         LOGIN_URL='users:login',
         LOGIN_REDIRECT_URL='apps:index',

plinth/modules/config/config.py

@@ -28,9 +28,11 @@ from django.template.response import TemplateResponse
 from django.utils import translation
 from django.utils.translation import ugettext as _, ugettext_lazy
 import logging
+import os
 import re
 import socket
 
+import plinth
 from plinth import actions
 from plinth import cfg
 from plinth.modules import firewall
@@ -77,6 +79,7 @@ class TrimmedCharField(forms.CharField):
 
         return super(TrimmedCharField, self).clean(value)
 
+
 def domain_label_validator(domainname):
     """Validate domain name labels."""
     for label in domainname.split('.'):
@@ -123,10 +126,22 @@ class ConfigurationForm(forms.Form):
 
     language = forms.ChoiceField(
         label=ugettext_lazy('Language'),
-        help_text=\
-        ugettext_lazy('Language for this web administration interface'),
-        required=False,
-        choices=settings.LANGUAGES)
+        help_text=ugettext_lazy(
+            'Language for this web administration interface'),
+        required=False)
+
+    def __init__(self, *args, **kwargs):
+        """Set limited language choices."""
+        super().__init__(*args, **kwargs)
+        languages = []
+        for language_code, language_name in settings.LANGUAGES:
+            locale_code = translation.to_locale(language_code)
+            plinth_dir = os.path.dirname(plinth.__file__)
+            if language_code == 'en' or os.path.exists(
+                    os.path.join(plinth_dir, 'locale', locale_code)):
+                languages.append((language_code, language_name))
+
+        self.fields['language'].choices = languages
 
 
 def init():