Depends how you define "in the kernel". In terms of what is compiled into the kernel image, it's C plus small pieces of assembler (both inline and in separate files). This proposal (if it ever happens) would be about adding Rust to that. If you mean the tree used to build the kernel, you could say it includes shell script, Makefiles, the Kconfig descriptions, and more. If you mean all the bits you need to have a very minimally working system which boots, you'd want to include all the languages used for the necessary userspace tools like the init system and udev (although the vast majority of that is also C and shell scripts).