Best foreign language to learn? (for an American)

Japanese grammar is not that difficult. It is actually much more logical than english.
However, I am going to say that if you are going to go into the world of physics, that you would be better off to learn a teutonic language like german. It seems that the majority of physics research is going on either in Europe or America. There also seems to be a lot of materials research going on in Japan, but this is more technologically based, and I am not sure how much application someone with a physics major would have.

However, I want to know, what sort of direction of physics do you want to go for? This is improtant to know, I think, before you can really decide what language you should look into.

Jam it back in, in the dark.