1. get some math course as #2 suggest. don't take too much math course each semester (suggest <= 2 every semester, maths course are difficult, especially those pure one..).
A couple of advanced stat / econometrics is highly recommended too.
an CS intro course (e.g. introduction of programming with C++) is good also, and you may also consider intermediate micro and macro (don't go to the adv one without good math / intermediate econ)
you may also consider an academics summer too (whatever summer school....)
2. don't suggest so. a 2nd language is good bonus, but it is not something necessary yet. most important point is......you don't know which language you need later now...(as you don't know which country you will go, even in Europe)
3. yes. will affect. don't take too much. I will suggest the normal workload of 4.