docs: def get_html_lang_tags(index_page):
      """
      Return `languages` stored in ``<meta>`` tags.
  
      ``<meta http-equiv="Content-language" content="cs">`` -> ``cs``
  
      Args:
          index_page (str): HTML content of the page you wish to analyze.
  
      Returns:
          list: List of :class:`.SourceString` objects.
      """
      dom = dhtmlparser.parseString(index_page)
  
      lang_tag = "content-language"
      lang_tags = dom.find(
          "meta",
          fn=lambda x: x.params.get("http-equiv", "").lower() == lang_tag
      )
  
      return [
          SourceString(tag.params["content"], "HTML")
          for tag in lang_tags
          if "content" in tag.params
      ]