code: def language_id(self):
          
          # (Formerly known as "rosetta_i18n_lang_code")
          lang_id = self.kwargs['lang_id']
          if lang_id not in {l[0] for l in rosetta_settings.ROSETTA_LANGUAGES}:
              raise Http404
          if not can_translate_language(self.request.user, lang_id):
              raise Http404
          return lang_id
docs: Determine/return the language id from the url kwargs, after
          validating that:
          1. the language is in rosetta_settings.ROSETTA_LANGUAGES, and
          2. the current user is permitted to translate that language
  
          (If either of the above fail, throw a 404.)