On an FAA PPL, you can fly an N-reg worldwide, and you can fly a G-reg worldwide.
I have no idea about a D-reg and so far nobody has answered this. One would need to look in the German legislation for foreign license privileges in a D-reg.
The French (F-reg) have some concessions for non-EU residents (I have a reference somewhere) but this appears to be rare in Europe.
Converting an FAA PPL into a JAA PPL is a lot of work - you have to sit all the written exams (7) and do another skills test. If you really want to fly Euro-reg planes, and have no ambition to do an IR, then get a JAA PPL instead.