If you want to be knowledgable on the level of a professional mechanic, Matthew's answer is the way to go. However, until you can find such a position, learning more about bikes will only help.
While I can't speak to resources in Germany, I can tell you that I've learned bicycle repair from three sources: from friends, asking questions at my bike shop, and reading online.
I have a friend who's a part-time bike mechanic, so that's obviously helpful. I try not to impose on him too much, but he enjoys talking about bikes--what rider doesn't?
Your shop: When you bring your bike in to be repaired, ask questions. See if they'll let you watch while doing the repair. (If they're secretive about what they do, perhaps finding a better shop should be a priority.)
Finally, the internet:
The most comprehensive bike site on the net is, without a doubt, the site put together by the late, great Sheldon Brown. It's in English, but the writing is good and easy to follow, and covers a broad range of topics. Some of these essays are parodies, most are serious, but all have good points to make.
Youtube has a great many bicycle repair videos, and that's how I've learned how to adjust a hub and use a chain tool. Can anyone speak to the quality (or existence) of any repair videos in German? While you wouldn't be able to follow along, not having a shop or tools, watching the procedures will help familiarize you with them, so that things will go more quickly when you're in a shop later on.
This very site. We have users all over the world, and there are many questions here on maintenance that have already been answered, and please feel free to ask more if you see a gap. We also have a Terminology index you can consult (and even add to).
Sorry I can't answer your question about local resources, but learn as much as you can on your own and you'll be better equipped to take advantage of local opportunities when they come along.