What languages to learn for maths? For people with "hands-on" experience in mathematical research, what languages are the most beneficial to learn? I know that many graduate programs require some degree of non-native lingual proficiency, so what would be the pros and cons of pursuing various languages?

(I realize that this question is a bit broad; please ask if anything needs to be specified.)
Most US-based PhD programs have, as a requirement for the degree, at least having reading proficiency in one of: French, German, Latin, Russian or, increasingly, Chinese. The first few for obvious historical reasons. In fact, except for Chinese, the impetus is predominantly historical nowadays; internationally, most cutting-edge mathematics is being published in English.
French for sure. Opens up a whole wide world. Much more opportunities. Non-francophone countries accept to do business in English.