Create index file of cached translations.

    Parameters
    ----------
    data_dir : pathlib.Path
        Cache directory location.

def _write_index_file(data_dir):
    """Create index file of cached translations.

    Parameters
    ----------
    data_dir : pathlib.Path
        Cache directory location.
    """
    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)