Unless you go for programmes that explicitly are in English only, no, it's not certain. I'm a CS masters student right now and it's a wild mix: fully German, German but with English slides, fully English, ... If a student requests it many will switch to English (if no-one objects), but you can't rely on it. You probably could complete it without speaking much German, but don't expect to be able to take all classes. Might be problematic if you need more points in a category than there are english lectures for it.

A friend of mine studies computer science at the University of Paderborn (he, too, has a B.S. and is studying for his master degree). I know that at least some of his courses are in English.

From what I know, Paderborn is a good university for CS, but I don't know if they have any researchers working in your preferred areas.

EDIT: http://www.uni-paderborn.de/en/

Not sure. Never studied in Germany. Just make sure to speak some real foreigner students.

If you dont have local local languages, I would avoid large countries such as France, Italy, Germany or even Poland. Smaller post-soviet countries have sort 'self-esteem issue' and they will be probably friendlier to English-only speaker.

No it is not. But there are indeed few Masters programmes which are instructed in English. The University of Göttingen is a good example. http://www.uni-goettingen.de/en/40954.html