What are the most useful languages to learn?

Aside from English and Spanish. This one I can't figure out myself because (for example) while Japanese is one of the most widely-spoken languages in the world, its only really spoken in japan(and tiny parts of brazil I think). I want to be a diplomat, ambassador or have some job along those lines so I'd like to learn languages that would give me the most 'bang for the buck' and be used in several countries across the globe. thank you :)