They run most anything from common to rare. The problem isn't the language, it is the years of cruft that never was fully documented. The people who made the early design decision are probably dead. Same problem any code you write today will have it is still in use in 50 years. Well maybe not quite as bad, if you paid attention you know some modern best practices that didn't exist then.

I had some colleagues who had worked on mainframe operating systems tailored for the airlines (PARS), and it's built upon TPF as I understand it. I don't know which language(s) it's implemented in, but I do recall them talking about assembler having a major role.

Java, C, C++ and Fortran, POSIX environments as well.

Then Algol, COBOL, RPG, NEWP, PL/I.