Well, I think for a lot of people, Spanish is a prerequisite. I agree that German, French, Russian, Japanese, and Chinese are all super helpful in international business, but I think maybe you should add Spanish to that list as well, it's at least worth some time. Also, I would suggest Korean because it is another attractive language for businesspeople. It is also hard, writing and reading aren't too difficult, but the grammar is quite different from European languages. Either way, what really matters isn't what to learn but how much you can realistically learn. You'll probably get farther in Spanish or French in a few months than several years in Chinese or Japanese. That's just my opinion though. No real right answer. Just find a language you are really good at and learn it thoroughly.