- 
frontend\views\layouts\main.php 
	         ...
                 $menuItems = [
                   ['label' => 'English', 'url' => ['/site/setlang&lang=en-US']],
                   ['label' => 'Deutsch', 'url' => ['/site/setlang&lang=de-DE']],
                   ['label' => '中文',  'url' => ['/site/setlang&lang=zh-CN']],
		  	
the url in the browser: index.php?r=site%2Fsetlang%26lang%3Dde-DE
   erro: Not Found (#404)
      the problem is suposted to be the procent encoding %26 and %3D. 
The %2F works fine like: index.php?r=site%2Findex
- works fine with:
 
<a href = "index.php?r=site/setlang&lang=de-DE" >Deutsch</a>
in the browser, url: index.php?r=site/setlang&lang=de-DE
I would say, it a bug, because yii interpretet the %26 not to & and %3d not to = respectively.