They generally can't speak French, German, or Japanese either. Unless you're one of the very very few legitimate Chinese-speaking players (They do exist, but even then they generally know a passable amount of either English or Japanese otherwise they can't get anywhere in the game) I doubt you'll have a hard time telling they're struggling with answering you.

Still, there's plenty of RMT operations run by English-speaking players anyway (a lot of big companies are run by US companies in one way or another), so that's not really a foolproof way of ensuring you're not supporting RMT or whatever.