If all you see with languages is the dollars you can make with an hour of time then learn whatever the next COBOL, C#, Java or Python. If you are focusing on solving problems, then learning another language, especially one that allows you to approach problems differently makes a lot of sense. I haven't used Prolog in anger in about 20 years. But the experience with declarative, logic programming has carried over to nearly everything I've done (you'll see state machines differently and see other logic patterns you can generalize). The resume isn't the be-all end all. Programming is a portfolio profession. You can prove your skill by showing your work - even if it's just some toy you wrote for fun on the weekend.