That is a good question to consider. I like Ben Halpern's answer.
I would not put on a résumé a language that I've learned for fun, since I've rarely done nothing more than some toy work with the language. So although I really like F# and D a lot (a whole lot of a lot!), I would not include it in my skill set.
If there was a position that used those languages, I'd still happily apply for the position even without that skill "under my belt". (I'm not in the market for looking for a new position. It's just hypothetical.)
I'm to the point in my career that I would also omit from my résumé any language that I never want to program in again. For example, I would not list Assembly, BASIC, FORTRAN, Pascal. Nothing egregiously wrong with those languages, just not ones I would want to work with anymore.
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