What language a mathematician uses is highly dependent on the field. I know of algebraists who use Maple, Magma, and Sage. I know numericists who use Fortran (even 77 ><), some who use Matlab, some who use C++ and some who use C (overlapping in many cases). I know of some mathematicians who use Python. I personally like functional languages, thought I am not familiar personally with any mathematicians who use them regularly (by this I mean purely functional languages... the few, the proud). I have seen some people use Mathematica, though I myself don't use it. I know of many mathematicians who try to avoid computers and programming languages.