You might be surprised how often multi-language programs appear. Basically all of modern day Python for starters. But also a number of important numerical libraries for C are actually written in Fortran.
> You might be surprised how often multi-language programs appear. Basically all of modern day Python for starters. But also a number of important numerical libraries for C are actually written in Fortran.
In my experience, the most common use case is C-based programs with some C++ sprinkled in (especially if you need the C code to be compiled exclusively by a C compiler in order to maintain C semantics).