well, at any given university in the us, there are usually many students from china, south korea, and india. at my university, just these three nationalities account for more than 50 percent of all international students. they are usually pretty well organized, have their own student associations, etc, so it is hard not to speak one's native tongue. on the other hand, if you come from iceland or lithuania, you have no choice but to speak english so you tend to improve it more.
another thing is the field of study. i know international students who, for example, study computer science or mathematics where there is no need to "talk" much. for five years it is sufficient to say "let x be any real number.... then it follows...." and write a proof on the board. students from those departments tend to suck at english, no matter what nationality.
