If one is not using a language, either on the job or for personal things, then I'm not sure why one would need to stay motivated at all. I also don't think you loose what you learned about the language - you may become rusty and not up-to-date on the latest stuff, but I suspect you could easily pickup some code and still understand it.

I think that if you like a language and it interests you, you'll have whatever motivation is needed to keep your fingers in it, whether by writing personal stuff or hanging out in places like PerlMonks. And, if you don't really like or enjoy the language, what's the reason for wanting to stay motivated about it?

Scott