Best foreign language to learn? (for an American)

Yes, a lot of Germans do speak English. Same goes for many of the germanic/scandinavian countries, probably because they're easier for English speakers to pick up on and vice-versa.

I don't know much about physics, but if Japan is a heavy hitter in that area, then I'd go with Japanese. Of course, many Japanese people speak some amount of English as well, so unless you think you'll be interacting with them a lot in your future career...I'd go with Spanish. It's not hard to pick up, it's growing in use in the US every day, and since it's Latin-based it makes figuring out a lot of the other common languages (ie: French, Portuguese, Italian) much easier.

Jam it back in, in the dark.