Found the problem, I had a small mistake in the German language tag. What I had was "de - DE" what I should have had was: "de-DE". Without spaces.
Note that by changing the language tag in the language manager, all articles and menu items that you set to that particular language will disappear. Just make sure that after you change the language tag you also check your menu items and article. If you don't they will not show up on the front end.