My impression is that the vast majority of mathematical articles written in the last 20 years are in English and that even foreign mathematicians try to write their better papers in English. It is, for better or worse, the only way to maximize the number of people who will read your paper.
So the only real need to read in a language other than English is to be able to read older papers. Which language you need depends pretty strongly not only in which subject you are interested in but the specific topics. For algebraic geometry, French can be very useful, but Italian can be, too. In differential geometry, the two languages that arise most often are French, German, and Russian. It really depends on whose work you want to read.
Overall, French is the most common language after English, because French mathematicians have been the most reluctant to switch to writing in English. For example, it is nice to be able to read the Seminaires Bourbaki.