English, Spanish, Mandarin covers the majority of the world as far as languages go. Russian wouldn't be bad if you're involved in business in Eastern Europe.
I'd be careful about using Russian in Eastern Europe. Most Eastern European countries, i.e Czech Republic, Hungary, have bad memories of Soviet occupation. Granted, it would be easier to get around with Russian rather than learning 8 or so different languages. But don't expect to recieve very warm responses using Russian throughout that region.
Also, to the OP, don't overlook French. Like Spanish, it's relatively easy and fun to learn, and it's still considered a sign of sophistication throughout the world. And it's always a surefire panty dropper.