function getLang(lang) {
    if (!includes(configs.langs, lang)) {
        const _navigator = window.navigator;

        lang = (_navigator.languages && _navigator.languages[0]) ||
            _navigator.language ||
            _navigator.userLanguage ||
            _navigator.browserLanguage || '';
    }
    return normalizeLang(lang) || 'en';
}
Returns the language code for user interface. The returned value is
ISO 639-1 code, but the exception is Chinese (zh-Hans or zh-Hant).
@param {string} lang - Language code to verify
@returns {string} Language code for user interface