locale: Anonymous users can set preferred language

Logged-in user flow is pending.

Check [How django discovers language preference]:
https://docs.djangoproject.com/en/1.10/topics/i18n/translation/#how-django-discovers-language-preference

Signed-off-by: Sai Kiran Naragam <saikiran.rguiiit@gmail.com>
Reviewed-by: Sunil Mohan Adapa <sunil@medhas.org>

plinth/forms.py

@@ -20,10 +20,15 @@ Common forms for use by modules.
 """
 
 from django import forms
-from django.utils.translation import ugettext_lazy as _
+from django.conf import settings
+from django.utils import translation
+from django.utils.translation import ugettext_lazy as _, get_language_info
 
+import plinth
 from plinth import utils
 
+import os
+
 
 class ServiceForm(forms.Form):
     """Generic configuration form for a service."""
@@ -48,3 +53,21 @@ class DomainSelectionForm(forms.Form):
         ),
         choices=[]
     )
+
+
+class LanguageSelectionForm(forms.Form):
+    """Form for selecting the user's preferred language """
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        supported_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)):
+                supported_languages.append((language_code, get_language_info(language_code)['name_local']))
+
+        self.fields['language'].choices = supported_languages
+
+    language = forms.ChoiceField(label='Language', choices=[])

plinth/templates/base.html

@@ -172,6 +172,11 @@
                 </ul>
               </li>
             {% else %}
+              <li>
+                <a href="{% url 'language-selection' %}" title="{% trans "Select Language" %}">
+                 <i class="glyphicon glyphicon-globe nav-icon"></i>
+                </a>
+              </li>
               <li>
                   <a href="{% url 'users:login' %}" title="{% trans "Log in" %}">
                     <i class="glyphicon glyphicon-user nav-icon"></i>

plinth/templates/language-selection.html

@@ -0,0 +1,38 @@
+{% extends 'base.html' %}
+{% comment %}
+#
+# This file is part of Plinth.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+{% endcomment %}
+
+{% load i18n %}
+
+{% load bootstrap %}
+
+{% block content%}
+
+    <h3> Choose your language </h3>
+
+    <form method="post">
+          {% csrf_token %}
+
+          {{ form|bootstrap }}
+
+          <input type="submit" class="btn btn-primary"
+                 value="{% trans "Save Changes" %}"/>
+    </form>
+
+{% endblock %}

plinth/urls.py

@@ -28,6 +28,7 @@ from . import views
 
 urlpatterns = [
     url(r'^$', views.index, name='index'),
+    url(r'^language-selection', public(views.LanguageSelectionView.as_view()), name='language-selection'),
     url(r'^apps/$',
         TemplateView.as_view(template_name='apps.html'), name='apps'),
     url(r'^sys/$', views.system_index, name='system'),

plinth/views.py

@@ -23,6 +23,11 @@ from django.core.exceptions import ImproperlyConfigured
 from django.template.response import TemplateResponse
 from django.views.generic import TemplateView
 from django.views.generic.edit import FormView
+from django.http import HttpResponseRedirect
+from django.conf import settings
+from django.shortcuts import render
+from django.urls import reverse
+from django.utils import translation
 from django.utils.translation import ugettext as _
 from stronghold.decorators import public
 import time
@@ -63,6 +68,30 @@ def system_index(request):
     return TemplateResponse(request, 'system.html')
 
 
+class LanguageSelectionView(FormView):
+    """View for language selection"""
+    form_class = forms.LanguageSelectionForm
+    template_name = 'language-selection.html'
+
+    def get(self, request, *args, **kwargs):
+        current_values = {'language': translation.get_language()}
+        form = self.form_class(initial=current_values)
+        return render(request, self.template_name, {'form': form})
+
+    def post(self, request, *args, **kwargs):
+        form = self.form_class(request.POST)
+        if form.is_valid():
+            selected_language = form.cleaned_data['language']
+            translation.activate(selected_language)
+            # set selected language in session
+            request.session[translation.LANGUAGE_SESSION_KEY] = selected_language
+            response = HttpResponseRedirect(reverse('language-selection'))
+            # send a cookie for selected language
+            response.set_cookie(settings.LANGUAGE_COOKIE_NAME, selected_language)
+            return response
+        return render(request, self.template_name, {'form': form})
+
+
 class ServiceView(FormView):
     """A generic view for configuring simple services."""
     clients = []