Far as I can see the German translation wasn't ready for the v3.6.7 release or it wasn't there when I downloaded the .PO files, but you can simply download the .PO file and replace it in app/content/languages and it should fix the missing strings right away.
Let me know if it works for you.
P.S. I've updated the v3.6.7 download with the missing strings. If you re-download v3.6.7 all the languages folder will contain the updated files.