With classes in English but really solid technical courses, you might want to consider one of:
Budapest Semesters in Mathematics
Aquincum Institute of Technology
Math in Moscow
The first two programs are both in Budapest, and the second one focuses on computing while the first one is (obviously) more focused on mathematics. If you have a theoretical bent these are good choices.
Comments
With classes in English but really solid technical courses, you might want to consider one of: Budapest Semesters in Mathematics Aquincum Institute of Technology Math in Moscow
The first two programs are both in Budapest, and the second one focuses on computing while the first one is (obviously) more focused on mathematics. If you have a theoretical bent these are good choices.