Fixed #21389 -- Accept most valid language codes

By removing the 'supported' keyword from the detection methods and only relying
on a cached settings.LANGUAGES, the speed of said methods has been improved;
around 4x raw performance. This allows us to stop checking Python's incomplete
list of locales, and rely on a less restrictive regular expression for
accepting certain locales.

HTTP Accept-Language is defined as being case-insensitive, based on this fact
extra performance improvements have been made; it wouldn't make sense to
check for case differences.

django/middleware/locale.py

@@ -1,7 +1,5 @@
 "This is the locale selecting middleware that will look at accept headers"
 
-from collections import OrderedDict
-
 from django.conf import settings
 from django.core.urlresolvers import (is_valid_path, get_resolver,
                                       LocaleRegexURLResolver)
@@ -21,7 +19,6 @@ class LocaleMiddleware(object):
     response_redirect_class = HttpResponseRedirect
 
     def __init__(self):
-        self._supported_languages = OrderedDict(settings.LANGUAGES)
         self._is_language_prefix_patterns_used = False
         for url_pattern in get_resolver(None).url_patterns:
             if isinstance(url_pattern, LocaleRegexURLResolver):
@@ -37,9 +34,7 @@ class LocaleMiddleware(object):
 
     def process_response(self, request, response):
         language = translation.get_language()
-        language_from_path = translation.get_language_from_path(
-            request.path_info, supported=self._supported_languages
-        )
+        language_from_path = translation.get_language_from_path(request.path_info)
         if (response.status_code == 404 and not language_from_path
                 and self.is_language_prefix_patterns_used()):
             urlconf = getattr(request, 'urlconf', None)