Switching languages and Language in URL ¶

I already wrote how translations work. Here I will show how language can be switched and saved into the URL. So let's add the language switcher into the main menu:

echo Nav::widget([
 'options' => ['class' => 'navbar-nav navbar-right'],
 'items' => [
  ['label' => 'Language', 'items' => [
    ['label' => 'German' , 'url' => \yii\helpers\Url::current(['sys_lang' => 'de']) ],
    ['label' => 'English', 'url' => \yii\helpers\Url::current(['sys_lang' => 'en']) ],
   ],
  ]

Now we need to process the new GET parameter and save it to Session in order to keep the new language. Best is to create a BaseController which will be extended by all controllers. Its content looks like this:

<?php
namespace app\controllers;
use yii\web\Controller;
class _BaseController extends Controller {
  public function beforeAction($action) {
    if (isset($_GET['sys_lang'])) {
      switch ($_GET['sys_lang']) {
        case 'de':
          $_SESSION['sys_lang'] = 'de-DE';
          break;
        case 'en':
          $_SESSION['sys_lang'] = 'en-US';
          break;
      }
    }
    if (!isset($_SESSION['sys_lang'])) {
      $_SESSION['sys_lang'] = \Yii::$app->sourceLanguage;
    }
    \Yii::$app->language = $_SESSION['sys_lang'];
    return true;
  }
}