code: def add_filter(func, language=None):
      
      if language not in _filters:
          _filters[language] = []
      _filters[language].append(func)
docs: Register filters for specific language.
      If language == None the filter applies for all languages.
      Filter will not apply for stop words in cache.
      :param func: callable
      :param language: string|None
      :return: