Well once you have a C API, it is relatively easy to call it from any other language.

The painful part is to have to go through a C API (modern languages can express much richer APIs and of course there are different constraints on the different runtimes, e.g. GC).

The annoying part is that each language adds overhead (its runtime). I wouldn't call it painful (I don't have much to do about it), I say "annoying" just because I would rather minimise the amount of code I ship.