public static function languagesLocaleCodes()
    {
        static $list;

        if ($list === null) {
            $list = [];
            foreach (static::codes() as $localeCode) {
                if ($languageCode = static::languageCode($localeCode)) {
                    $list[$languageCode][] = $localeCode;
                }
            }
        }

        return $list;
    }