Hi, I'm not an expert, but as far as I know there is no clear rule and it can depend on local rules and ownderships. But usually it is not allowed to stay in tent in a forrest/nature. My impression is, that you usually don't get in trouble if you respect the nature (no fire/no waste). In nature reserves it is definitely forbidden.
Unfortunately there is only a very little number of places, where you officially can stay in a tent (except commercial camping sites). Some free nature campsites can be found here on the website (they are mentioned in my lists of accomodations) and some others can be found here:
1nitetent.com/
Please keep in mind that February is one of the coldest months of the year in Germany and it can become very could, especially in the middle range mountains.
Info: I moved your post to the english section of our forum.