I need to learn Greek - but I want to learn it with my own language German. When will this be poss.
There is no fixed time point. The Duolingo courses are provided by private people which create them in their free time. There will be a course only if there are enough greek native speakers which know German well enough to create a course from German to Greek. Maybe, this will never happen... If you really want to learn Greek, you should keep on going with the English Greek course.
This is a poor fix, but mondlylanguages (website and app) allows learning from any of its included languages to any of them, and since it has both Greek and German, you could use that. Bear in mind that it's mainly good for vocabulary expansion and some sentence structure, with no grammar, so you'd have to find that elsewhere.