Simplify language chooser (#328)

* Simplify language chooser

- Allows usage of i18n_patterns with prefix_default_language=False
- Change templatetag from simple tag to an inclusion tag
- Reduces complexity by relying on Django's behavior in the
  set_language view: it will translate any url passed as 'next'.
  This behavior has been present since Django 1.9.
- Remove individual forms for each language

Fixes #327

Reference: https://github.com/django/django/commit/aa5ab114e34645823b219f9a866fd82fc26b427b

* Add check to ensure LocaleMiddleware

* Remove check in favor of silent warning

* Fix template tag tests

admin_interface/templates/admin/base_site.html

@@ -186,7 +186,6 @@ flat-theme admin-interface
 {{ block.super }}
 {% get_admin_interface_theme as theme %}
 {% if theme.language_chooser_active %}
-    {% get_admin_interface_languages as languages %}
-    {% include "admin_interface/language_chooser.html" %}
+    {% admin_interface_language_chooser %}
 {% endif %}
 {% endblock %}

admin_interface/templates/admin_interface/language_chooser.html

@@ -1,18 +1,15 @@
 {% load admin_interface_tags %}
 
-{% if languages %}
+{% if set_language_url %}
 <div class="language-chooser {% if theme.language_chooser_control == 'minimal-select' %}minimal {% endif %}">
-    {% for language in languages %}
-    <form class="language-chooser-hidden-form" id="language-chooser-hidden-form-{{ language.code }}" action="{{ language.activation_url }}" method="POST">
+    <form class="language-chooser-select-form" action="{{ set_language_url }}" method="POST">
         {% csrf_token %}
-        <input name="language" type="hidden" value="{{ language.code }}">
-    </form>
-    {% endfor %}
-    <form class="language-chooser-select-form">
-        {% csrf_token %}
-        <select name="language" onchange="document.getElementById(String('language-chooser-hidden-form-' + this.value)).submit();">
-            {% for language in languages %}
-            <option value="{{ language.code }}" {% if language.active %}selected{% endif %}>{% if theme.language_chooser_display == 'code' %}{{ language.code|upper }}{% elif theme.language_chooser_display == 'name' %}{{ language.name }}{% endif %}</option>
+        <input type="hidden" name="next" value="{{ next }}">
+        <select name="language" onchange="this.form.submit();">
+            {% for code, language in LANGUAGES %}
+            <option value="{{ code }}" {% if code == LANGUAGE_CODE %}selected{% endif %}>
+                {% if theme.language_chooser_display == 'code' %}{{ code|upper }}{% elif theme.language_chooser_display == 'name' %}{{ language }}{% endif %}
+            </option>
             {% endfor %}
         </select>
     </form>

admin_interface/templatetags/admin_interface_tags.py

@@ -1,6 +1,7 @@
 import datetime
 import hashlib
 import re
+import warnings
 
 from django import template
 from django.conf import settings
@@ -16,45 +17,43 @@ from admin_interface.models import Theme
 register = template.Library()
 
 
-@register.simple_tag(takes_context=True)
-def get_admin_interface_languages(context):
+@register.inclusion_tag("admin_interface/language_chooser.html", takes_context=True)
+def admin_interface_language_chooser(context):
     if not settings.USE_I18N:
         # i18n disabled
         return None
     if len(settings.LANGUAGES) < 2:
         # less than 2 languages
         return None
+    if "django.middleware.locale.LocaleMiddleware" not in settings.MIDDLEWARE:
+        warnings.warn(
+            "Language chooser requires 'django.middleware.locale.LocaleMiddleware' "
+            "in your MIDDLEWARE to work.",
+            stacklevel=1,
+        )
+        return None
     try:
-        set_language_url = reverse("set_language")
+        context["set_language_url"] = reverse("set_language")
     except NoReverseMatch:
-        # ImproperlyConfigured - must include i18n urls:
-        # urlpatterns += [url(r'^i18n/', include('django.conf.urls.i18n')),]
+        warnings.warn(
+            "Language chooser requires Django's `set_language` view: "
+            "`urlpatterns += [url(r'^i18n/', include('django.conf.urls.i18n'))]`.",
+            stacklevel=1,
+        )
         return None
     request = context.get("request", None)
     if not request:
         return None
+    context["LANGUAGES"] = settings.LANGUAGES
+
     full_path = request.get_full_path()
     admin_nolang_url = re.sub(r"^\/([\w]{2})([\-\_]{1}[\w]{2,4})?\/", "/", full_path)
-    if admin_nolang_url == full_path:
-        # ImproperlyConfigured - must include admin urls using i18n_patterns:
-        # from django.conf.urls.i18n import i18n_patterns
-        # urlpatterns += i18n_patterns(url(r'^admin/', admin.site.urls))
-        return None
-    langs_data = []
+
     default_lang_code = settings.LANGUAGE_CODE
     current_lang_code = translation.get_language() or default_lang_code
-    for language in settings.LANGUAGES:
-        lang_code = language[0].lower()
-        lang_name = language[1].title()
-        lang_data = {
-            "code": lang_code,
-            "name": lang_name,
-            "default": lang_code == default_lang_code,
-            "active": lang_code == current_lang_code,
-            "activation_url": f"{set_language_url}?next=/{lang_code}{admin_nolang_url}",
-        }
-        langs_data.append(lang_data)
-    return langs_data
+    context["LANGUAGE_CODE"] = current_lang_code
+    context["next"] = admin_nolang_url
+    return context
 
 
 @register.simple_tag()