Students planning a summer or a semester abroad should contact the Office of International Studies. Their programs page describes how to choose a location. A particular consideration is whether there are suitable mathematics courses at a prospective institution. Mathematics courses at foreign universities can be evaluated to see if they are equivalent to courses at Northeastern University. Some universities already have exchange programs with Northeastern; here is a list.
One international program that has been designed especially for math majors is the Budapest Semesters in Mathematics.