I agree. Functional programming should not be viewed as a tuxedo that we dust off and use when we want to be "fancy programmers." It's simply a tool in the toolbox.

The best language for learning functional programming is the one that de-mystifies it. And since you can use many (most?) popular languages to code in a functional style, I'd say, take whatever language the students have been using, and teach them a few functional programming techniques with it.