APL is super powerful, I got to the level where I could 'read' (decode) it but I've never managed to get proficient at a level where I could write a significant chunk of code in it. Say a line or two ;)

FORTH is extremely easy to get into, a very powerful metaphor that has a ton of parallels in other languages, even if you don't realize it. There are strong connections between FORTH, LISP and shell programming languages to the point that they are in a manner of speaking different ways to express the same thing. Of those three I think LISP is the most complete and feature rich, and FORTH probably the least because it is much more barebones. But that upside of that is that it also requires absolutely no supporting infrastructure, the most basic access to a machine at the hardware level is enough to bootstrap to a usable system in an absolute minimum memory profile.