I'm not learning any language right now, but have learned a fair share over the course of the past few years.
My general criteria is not to learn a language that is conceptually similar to something you already know unless there are professional advances to be had there. If all you know is Python, C might be good because it's lower level. Lisp or Haskell or O'Caml might be good because you'll have to program with different paradigms, etc. If I were coming from Python, Ruby wouldn't be my next choice unless I needed it for a job.
My general criteria is not to learn a language that is conceptually similar to something you already know unless there are professional advances to be had there. If all you know is Python, C might be good because it's lower level. Lisp or Haskell or O'Caml might be good because you'll have to program with different paradigms, etc. If I were coming from Python, Ruby wouldn't be my next choice unless I needed it for a job.