First, have a comprehensive enough userspace that it makes sense... You'll need a C standard library at the
very least, and likely a bunch of other support libraries.
Second, port the language's compiler(*)/interpreter/runtime to your OS. Depending on how comprehensive your userspace is, how similar it is to other OS's that the language supports and how much of a priority the original developers put on portability this can range from "trivial" to "several years of work".
The wiki has an article on
Porting Python, which should at least cover the basic concepts (although the exact steps will be different) for porting similar software.
If you're talking about using other languages in your kernel, that's a bit different. Some highly-portable languages (such as those designed to work on "embedded" systems) like Lua can be adapted for kernel use fairly easily, but anything with a substantial runtime is likely to be quite difficult...
(*) For compiled languages, you may not have to port the compiler to your OS itself, but just re-target it to act as a cross-compiler running on another OS (i.e. your development environment).