Wouldn’t it by nice if Yii2 has a standard configuration with:
$config = [
...
// see http://php.net/manual/en/book.intl.php
'language' => Locale::acceptFromHttp($_SERVER['HTTP_ACCEPT_LANGUAGE']),
// standard language for messages and views. Do not change from en
‘sourceLanguage’=>‘en’,
...
In this way the language locale is derived from the browser.