C++ and Python pretty much run everything, everywhere. Standard, open languages are hard to beat when you want to fully control your development stack and not worry about future control issues. I wish go was an ISO standard like C++, I'd be more interested if it was.
RPG was more oriented towards mini-computers than mainframes, from what I recall. It's niche was the IBM S/36, S/38, AS/400 and iSeries boxes. I'm sure you probably could get RPG for your S/360 or S/390 or whatever, but from what I've seen over the years, it was mostly COBOL, PL/I, and MVS Assembler on the mainframes.
