kernel thread

In effort to have a better understanding of System Architecture (main course next semester) I'm writing a little kernel.
It will have at least one runtime for a language that is not C or C++.

What language do you think I should pick, and if the runtime requires a garbage collector (D, Go, Python) should I add one to the kernel as well?

The easiest language to implement at that level is Forth, especially if you know well enough Assembly to write a kernel.
Read this to get an idea of how to do it: http://git.annexia.org/?p=jonesforth.git;a=summary

I don't think you should go with hard-to-parse languages. If you want an high-level language, go for a simple Lisp. You don't have to implement the whole CL or R5RS (with continuations).

A GC built in the kernel for the GCed language is desiderable.