In my experience it is like learning a spoken language. If you know a language in the same family then learning the new language is relatively easier (e.g, Java -> Scala, Lisp -> F#). I also think the ease of learning a language is proportional to the ratio of expressiveness of the language you are coming from to the expressiveness of the language you are going to (e.g., Lisp to F# might have a much easier time than F# to Lisp).