code: def _write_index_file(data_dir):
      
      cached_words = [
          w
          for w in _get_words(data_dir)
          if data_dir.joinpath("translations/{}.html".format(w)).is_file()
      ]
  
      content_str = _create_index_content(cached_words)
      html_string = HTML_TEMPLATE.replace("{% word %}", "Index")
      html_string = html_string.replace("{% content %}", content_str)
  
      filename = data_dir.joinpath("index.html")
      save_file(filename, html_string, mk_parents=True)
docs: Create index file of cached translations.
  
      Parameters
      ----------
      data_dir : pathlib.Path
          Cache directory location.