Concerning language proficiency, I am pretty sure you would have to show a command of german to allow you to pilot a plane and deal with ATC in that language, the background is as follows:
In germany there are 3 different Radio Licences for Aviation - BZF I, BZF II and AZF.
The BZF II is the basic licence and allows a Pilot to operate a radio station in an aviation context within German airspace in German language ONLY.
BZF I allows the Pilot to operate a radio station in an aviation context in english language - note no restriction on German airspace, hence if you're a PPL and want to fly outside of Germany, you need the BZF I.
Both of the above are identical in terms of learning - the BZF exam is a multichoice theory paper (in German!) with a pass rate of at least 75% required followed by a simulated take off and landing from an international airport including hand off to ground, taxing to and from the apron. The difference between the two is that with BZF II both the take off and landing are in German whereas with the BZF I, one of the take offs OR landing will be conducted in English, the other in German. Also, you will be given an english text and be requested to translate this into German.
AZF is the general licence and is required by ALL commercial aircrew - I don't have this licence hence I can't comment on the learning required, nor do I know whether and what any limitations would be, going straight for the AZF but nearly all flying schools train the PPL either for BZF II or I, not AZF.
As I'm a native english speaker, I asked the question whether I could just go for the BZF I and talk just in english, forget about the german phraseology but this is NOT possible, you NEED to be able to answer the german test paper correctly plus you need to be able to do an approach or landing from in German language and be able to translate an english text into german.