Julia is mostly (> 65 %) written in Julia. There's some C (20 %, bootstrap etc.), some C++ (8 % llvm interface), some scheme (4 %, parser). I think it's one of the main points, that Julia should be strong enough that such things _can_ be written in itself.

Go was initially written in C. I think many consider C++ to be unnecessarily complicated and avoid it at the lowest level.

Btw.: Do you know the developer documentation? Just in case: http://docs.julialang.org/en/latest/#Developer-Documentation...