For a real world project pick a suitable language that will not only help you to implement whatever you have to implement but also to maintain the project. Also you should be able to find other programmers knowing that language if you need to. Maybe even use the language you already know even if it is not the newest and hottest stuff.
But for private or side projects use whatever enlightens you. Those projects should be fun and it's always good to learn new things on the way. I even think you don't have to learn all new languages in depth but just do a small project and reflect if it is something that gets you somewhere. You probably wont write a big business application in Smalltalk but all that message passing, awesome. Seriously have a look at Smalltalk.
But for private or side projects use whatever enlightens you. Those projects should be fun and it's always good to learn new things on the way. I even think you don't have to learn all new languages in depth but just do a small project and reflect if it is something that gets you somewhere. You probably wont write a big business application in Smalltalk but all that message passing, awesome. Seriously have a look at Smalltalk.