Perhaps Prolog and LISP are something any developer should learn. Greenspun's tenth rule (citing Wikipedia):

"Any sufficiently complicated C or Fortran program contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp."

And the same is somewhat true for many situations, not just C or Fortran programs. It may of course be, that you never really worked on projects, which would benefit from embedded LISP-like language, but by no means it's a waste of time.

Same for logical programming (not necessarily Prolog). This really enhances one's horizons.

Good, that you are still ready to learn miniKanren, Factor or Forth, Julia, Elixir, Haskell, ...