One instance of this phenomenon is Knuth's TeX program, written in a dialect of Pascal which Knuth calls Pascal-H, accepted by Charles Hedrick's compiler: I be

One instance of this phenomenon is Knuth's TeX program, written in a dialect of Pascal which Knuth calls Pascal-H, accepted by Charles Hedrick's compiler: I believe the sources for the compiler can be found here, https://github.com/PDP-10/rutgers-pascal/ https://github.com/PDP-10/rutgers-pascal/ And at some point Knuth had modified the GNU pascal compiler to compile modified TeX sources. But in practice everyone just translates from Pascal to C, via WEB2C. Programs like XeTeX are even writing new Pascal-H code, which gets translated to C, and seems unlikely to have ever been compiled in the original language.