OlafHimmelberg I had not only the German language package deactivated but also the English, I think that's where the error lies.
Yes, you need at least one language pack installed and it's wise not to remove or disable the English language pack as a fall back. It's better to have an English description than cryptic strings like core.forum.header.sign_up_link
. If you still see these cryptic strings you probably just need another php flarum cache:clear
.