So I have some time on my hands and I might as well learn another language. I'm currently fluent in English and Vietnamese, proficient (4 years) in Spanish, and basic (1 year) in French. I plan to improve my French and pick up another language. Since I only care about math, I'm thinking German, Russian, Mandarin, Latin, or (Ancient) Greek. 1. German and Russian are standard along with French. But I don't see them being useful since papers are rarely written in those languages anymore (while the French seem more obstinate). 2. Mandarin seems useful when China takes over. Though on a more serious note, there doesn't seem to be many international collaborations with the Chinese (what's even going on over there?) and breaking the language barrier could certainly help. 3. Latin and Greek seems useful in general and would help me brush up on my classics. Not to mention that I get to be more pretentious in my papers by adding obscure Greek symbols and Latin phrases (who wouldn't love to do this?). Just as a note, I'm definitely planning to learn Mandarin, Latin, and Greek at some point in my life. What languages would you personally recommend? And just out of curiosity, if you had time for another language, what would you want to learn?