Forth probably even more, since most things that are statements in other languages can be defined inside Forth, and you can end up easily building mutually incomprehensible DSLs even if they're technically runnable by the same interpreter.
Forth was the other example that came to mind. And possibly Haskell and Scala. But Lisp is the one that's well known with language extensibility as an advertised feature.
or SQL for that matter.
Same same, but different.
_Users_ can't redefine keywords and grammar in SQL, can they? All you can do is create data structures, and write functions and procedures.
Yes, you can write code that creates functions and procedures; it might even be possible to write code that modifies itself, but that still would be essentially different from what one can do in Lisp and Forth.