Re: Microfocus Cobol
The MF COBOL compiler is indeed written in COBOL, but it compiles to INT code, an intermediate form which runs on a virtual machine, mostly written in C.
There's also the code generator which emits GNT code, native code for the platform it's on, with a common object file format across systems, also loaded by the C runtime. That's also written in COBOL iirc, but it's been a long time so I could be wrong on that.
Reference: I used to work on the C runtime.