I mean, how "extremely difficult" are we talking? So long as it isn't esoteric, it can't be so difficult... otherwise no one would learn it, and it would become by definition esoteric...

I guess "real hard but still non-esoteric" could include * Functional ones like OCaml or Haskell (so I hear... "What are these monads you speak of?") * Super legit ones like C/C++ for systems programming * Rare and majestic ones like Lisp * Ones that require outside math knowledge like Julia or Fortran * Really new ones that don't have many packages / docs / community members yet