@coolboymew @Moon You can't switch Assembly languages on the same hardware architecture as each architecture has its own instruction set. I assume you are talking about switching from Assembly to C.
From what I heard, Mario's Face was programmed in C++ while everything else was programmed in C and Assembly. It was also programmed by the only foreigner on the team, Giles Goddard. Maybe you can contact him on Twitter for clarification.
@coolboymew @Moon Also, I expect SM64's source code to be a mess anyway because despite the C programming language being the primary language for the N64's API. It's different from traditional C code. In most C programs, the first function to be executed is main(). Apparently, the N64 uses a function called boot() when whatever game/software is being executed. There's a bunch of other stuff as well. The N64 was difficult to develop for compared to the PC and PlayStation, but not as difficult compared to the Sega Saturn.