Do language skills determine ones success in science? Or IQ plays more important role?

I scored 3 As (15 points) in A-level from Edexcel but now I am doing my degree in German language. Both English and German are not my mother tongue, but I did science and maths in English during my school days. I have a schoolmate whose English is noticeably poorer than mine but he performs much much better than me in the lecture(in German), in other words, I feel like I'd become dumb and it takes me long time to understand new maths theorem in German. Well, both of us are male, same race and score the same grade in science(A-level) and German language. I know this question is very subjective but I would like to hear from your opinions. This really baffled me very much.