setLocale() doesn't change the language set by browser

I still don't know if setlocal() should change the selected language used by the language files or if it was just my immagination...

So I finaly did manualy insert the language variable in all the lang() functions like that:

Code:

<?= lang('Titles.titleStundenplan', [],$language)?>

The only problem now is for the validation error language who strictly follows the user properties set in browser. I don't know how to change that manualy.

any help would be apreciated...