In chapter one of "The Pragmatic Programmer" the first learning goal is:
Learn at least one new language every year. Different languages solve the same problems in different ways. By learning several different approaches, you can help broaden your thinking and avoid getter struck in a rut. [...]
To achieve this over a career, the list of languages is likely to get quite long (particularly if you do not want to "progress" into management). Clearly the education of a programmer (or whatever form) is going to get you started with a core of commercially useful languages (the usual list from job posting: C, C++, Ruby, Python, JavaScript, C#, Java, VB, ...). Additionally a formal or informal learning programme is likely to have covered functional approaches (via something like Haskell, LISP or an ML derived language)
But once a reasonable subset of that list is learned- what's next, and why?