For a math major, French, German, or Russian could be a helpful foreign language, since math research papers are often written in those languages (mostly an issue at the PhD level).
At some universities, math majors are required to take some science courses (see the ASSIST reports), so you may want to choose your science courses for GE purposes with those requirements in mind.
You can get an unofficial assessment about your math placement with these quizzes: http://www.math.buffalo.edu/rur_index.html