Symbolics had C, Fortran, Pascal, Ada and Prolog implementations. The promised benefit of a single-language userspace is in simplifying the tools and debugging and/or making them more powerful. The Linux ABI, ELF, DWARF, linkers, make, and all the awful POSIX stuff is huge and hugely complicated, but you don't really get a lot for it. The other alternative are systems like Plan 9 and software from http://suckless.org/