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.

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
    ]