I have set up my org-mode so that I use variable pitch fonts for most of the items and fixed pitch fonts for verbatim and src blocks.

This works as expected for languages babel knows by default, for example emacs-lisp, python and c++.

However, I get variable pitch fonts for when I use an unrecognized language, such as nim (in my case) or R. 

This seems to be an issue related to faces and not blocks as a whole, since I can still run both "internal" and "external" languages: the ob-nim package works upon running the block via C-c C-c