At some point in your programming career, you should learn an ALGOL-descendant (C, C++, C#, Java, etc), a Lisp-descendant (CL, Scheme, etc), A Prolog-descendant

At some point in your programming career, you should learn an ALGOL-descendant (C, C++, C#, Java, etc), a Lisp-descendant (CL, Scheme, etc), A Prolog-descendant (Prolog itself, maybe SQL as a programming language), A Forth (better yet, read Starting Forth by Leo Brodie and write your own), an ML-descendant (like Haskell) and an APL-descendant like J. Nearly every language leans heavily upon one of these foundations.