You ask about "languages", plural, which is a good thing, since knowing more than one language is better for multiple reasons. Since this is a forum for Mac, iOS, aTV, watch, etc., I would say a mix of C, Obj C, Swift, and Python would be a good base set (maybe starting in the reverse order). From there you can go on to more exotic specialty languages, such as Metal, MatLab, Mathematica, Rust, Verilog, Fortran, and etc. If you want to teach, maybe add Lisp and Logo, or even Basic.