If I could only work with one language, then I'd have to say Forth. While I don't know this one nearly as well as any of the others, this one allows you to wor

If I could only work with one language, then I'd have to say Forth. While I don't know this one nearly as well as any of the others, this one allows you to work at the assembly instruction level up through the level of abstraction of languages like Python/Javascript. Lisp on LispMachine hardware would offer you the same sort of advantages. The C and D languages look like they might also be fairly good choices. Everything else doesn't seem to give you the flexibility to go down to the metal while also having the ability to move up to much higher levels of abstraction.