Some good points have been made.
I would add that learning languages you won't use in production work can be of value
To better appreciate and absorb the arguments and methods in texts and papers that will improve programming ability in languages I do use for production work (e.g. MIX/MMIX for Knuth's Art of Computer Programming; RATFOR for Kernighan and Plauger's Software Tools; I still use some ALGOL-based syntax for some pseudocode although I never wrote runnable code in ALGOL outside University)
To be able to check or prototype programs that will be written in a different language (e.g. some routines for numerical computing in C can be quickly checked or scaled using languages that have appropriate functionality built in such as Fortran, Python or Haskell)
Learning a new language can give insight as to how it could be used to more easily solve problems that were put to one side because of time or complexity constraints.
