code: def ref_lang(self):
          
          ref_lang = self._request_request('ref_lang', 'msgid')
          if ref_lang != 'msgid':
              allowed_languages = {l[0] for l in rosetta_settings.ROSETTA_LANGUAGES}
              if ref_lang not in allowed_languages:
                  raise Http404
          return ref_lang
docs: Return the language id for the "reference language" (the language to
          be translated *from*, if not English).
  
          Throw a 404 if it's not in rosetta_settings.ROSETTA_LANGUAGES.