Chinese is the language to go right now. Although more and more Chinese are learning English, the truth of the matter is that in business it always helps to know what the other person is thinking and why they react the way they do. Be warned though, it is a hard language. I have tried and fail many many times.
Chinese just because it's the most widely spoken language in the world at this moment. And yes, it beat English in a ratio of like 2:1. Korean doesn't even make it to the top 10. It is tough as hell to learn though, but a great challenge nonetheless.