[ffriend]

The two language problem is more specific to scientific computing where literally every popular library is written in more then one language - fast C/C++/Fortran and convenient high-level R/Matlab/Python. NumPy, SciPy, Caffe, Theano, Tensorflow to name a few. I once had to rewrite Matlab + C code (for face tracking) to pure Julia - not only the resulting code was almost twice smaller, it also ran ~20% faster.

[dnautics]

I wrote a library in c to replace a Julia one I wrote (and make it more portable) and found that the c version was slower! (Couldn't inline functions as easily in c and was also being more parsimonious about memory usage in c)