Django language middleware. Detect the user's browser language settings and activate the language. If the default language is not supported, try secondary options. If none of the user's languages are supported, then do nothing.

middleware.py

class LanguageMiddleware(object):
    """
    Detect the user's browser language settings and activate the language.
    If the default language is not supported, try secondary options.  If none of the
    user's languages are supported, then do nothing.
    """

    def is_supported_language(self, language_code):
        supported_languages = dict(settings.LANGUAGES).keys()
        return language_code in supported_languages

    def get_browser_language(self, request):
        browser_language_code = request.META.get('HTTP_ACCEPT_LANGUAGE', None)
        if browser_language_code is not None:
            logging.info('HTTP_ACCEPT_LANGUAGE: %s' % browser_language_code)
            # Split the string by , and split each item by ;. Sort out all strings which start with q=.
            languages = [lan for piece in browser_language_code.split(',')
                         for lan in piece.split(';') if not lan.startswith('q=')]
            for language in languages:
                language_code = language.split('-')[0]
                if self.is_supported_language(language_code):
                    return language_code
                else:
                    logging.info('Unsupported language found: %s' % language_code)

    def process_request(self, request):
        language_code = self.get_browser_language(request)
        if language_code:
            translation.activate(language_code)
            
            
class LanguageMiddlewareTest(BaseMiddlewareTest):

    language_middleware = LanguageMiddleware()

    def test_parse_http_accept_language(self):
        request = self.factory.get('/')
        request.META['HTTP_ACCEPT_LANGUAGE'] = 'de-DE,de;q=0.8,en-GB;q=0.6,en;q=0.4,en-US;q=0.2'
        language_code = self.language_middleware.get_browser_language(request)
        self.assertEqual(language_code, 'de')

    def test_parse_http_accept_language_not_set(self):
        request = self.factory.get('/')
        language_code = self.language_middleware.get_browser_language(request)
        self.assertIsNone(language_code)

    def test_parse_http_accept_language_empty(self):
        request = self.factory.get('/')
        request.META['HTTP_ACCEPT_LANGUAGE'] = ''
        language_code = self.language_middleware.get_browser_language(request)
        self.assertIsNone(language_code)

    def test_parse_http_accept_language_unsupported_language_fallback(self):
        """if language is not supported, try the next one"""
        request = self.factory.get('/')
        request.META['HTTP_ACCEPT_LANGUAGE'] = 'jp,de-DE,de;q=0.8,en-GB;q=0.6,en;q=0.4,en-US;q=0.2'
        language_code = self.language_middleware.get_browser_language(request)
        self.assertEqual(language_code, 'de')

    def test_parse_http_accept_language_unsupported_language_fallback(self):
        """if language is not supported, try the next one"""
        request = self.factory.get('/')
        request.META['HTTP_ACCEPT_LANGUAGE'] = 'jp'
        language_code = self.language_middleware.get_browser_language(request)
        self.assertIsNone(language_code)