Hello!
If I remember well Germany has those mill. low flying areas 1 to .. as shown on the Jeppessen maps.
There are none in that part of Germany that he intends to visit. And they are not restricted airspace anyway for daytime VFR flying. But since you may encounter low-flying miltary traffic anywhere in Germany, the official advice is to stay above 2000ft AGL whenever weather and airspace permits. (There used to be a military corridor all along the black forest right across the intended track, but that has disappeared over ten years ago.)
And some(most) fields have German R/T only.
I have no statistics, but I would say that 3/4 have German and English RT. It is written on the Jeppesen/Bottlang charts what languages are spoken. And every airfield/airport with a control zone or class F airspace must provide English RT.
Greetings, Max