It seems that knowing French is useful if you're an algebraic geometer. More generally, I've sometimes wished I could read German and Russian so I could read papers by great German and Russian mathematicians, but I don't know how useful this would actually be. What non-English languages are good for a generic mathematician to know? Are there specific languages associated to disciplines other than algebraic geometry?
(This question is a little English-centric, but I figure it's okay because this website is run in English.)
Best Answer
C and LaTeX. Speak them like it's the mother tongue.