def METHOD_NAME(self, lang):
    fallback = {'uk': 'ru'}
    for lang_part in lang.split("_"):
        lang_part = lang_part.lower()
        lang_part = fallback.get(lang_part, lang_part)
        if os.path.isfile(os.path.join(base_path, 'lunr-language', f'lunr.{lang_part}.js')):
            return lang_part