i had been having a bug, where my system messages are shown in the wrong language. I have got one simple question: How do i set the system message to always be e.g. english or german without adding &uselang=de to the URL. Is there a way to permanently change it in the local settings?
Yes, you need to change it in your file LocalSettings.php. For example, if you want to use Brazilian Portugese change it to:
$wgLanguageCode = 'pt-br';
For more info, see the manual: Manual:$wgLanguageCode