I expect the JVM to still be going; at least one of Clojure or Scala will probably continue growing for a long time, and there's room for both. It's also possible another Algol-esque language that isn't Java, but is syntactically closer to Java, making people more comfortable, will arise. At some point one of them may even become the "primary" JVM language, with Java hanging on as legacy, but due to the fact Java just compiles to the JVM, there won't be any reason to get rid of it, as the maintenance effort is virtually 0. I expect C# to probably still be going. I'm not sure anything can actually kill C, even though something really should.

C++ I can see going either way; it is a freakishly complicated language (and getting more complicated with every standard that comes out) that IMHO was primarily kept aloft by being the "default" language in Windows for a very long time. I expect Rust will be most of the way through eating it alive in 15 years. Or something like Rust, but Rust is looking pretty solid. And if you want a multithreaded systems language... and you will... I suspect in practice it's going to be an order of magnitude easier to program in than C++ in another three or four years. Maybe Rust will even be eating C. (About damn time!) Someone will probably be writing an OS kernel in Rust. Possibly one that is driver-compatible with Linux, or even just straight-up a Linux-compatible kernel.

If Google doesn't drop the ball, Go will probably be mature and still going, and probably the primary reason why Ruby and friends are considered legacy code. Go even today very nearly replaces the scripting languages in flexibility (though definitely not quite), but brings back stronger typing and better performance. A few more tweaks to it over the next 5 years and it'll be hard to see why I'd start a new project in a "scripting" language. (Part of the reason I think this is that I observe that in the past 15 years, no language has penetrated into the A-list without some sort of major corporate backer. While there's plenty of other possibilities from a technical point of view, and while Google is not very aggressively pushing Go, I still think it's more support than any other language has or is likely to have in the near future. Even if they just passively continue to do little more than employ the authors, this is likely to continue floating up the ranks.)

And I still sort of expect something to come up that is not Haskell, but is closer to Haskell than anything currently existing. Or possible Haskell simply keeps plugging along and manages to become a very solid B-list language. I doubt it'll ever break into the A-list "nobody got fired for choosing C/C++/Java/C#" level of popularity, but it might settle firmly into where "it's pretty hard to get fired for choosing Python/Ruby/PHP" is today. (It's brushing B-list status now, but it's got a ways to go before I'd call it solid B-list.) I also think there's a good chance it will acquire a reputation as being the tool of choice for tackling truly complicated systems. (Though Rust will eat into the bottom end of that a bit too, they're currently retreating from some of the tools that Haskell has that may allow it to acquire this reputation. I think Haskell will still retain a strong advantage here.)

And finally, some sort of wildcard. 15 years ago I doubt anybody would have predicted Haskell being where it is now, even Haskell users (if you can even call it the same l. New languages shall continue to bubble and froth, but at least one and probably two will either emerge from the froth and become a B-list language. Probably at least one of them will be a aggressively "practical" language that will be a reaction to the success of "excessively-academic" Clojure, Scala, Rust, and the Haskell-like I hypothesize above, and follow a hype curve very similar to Ruby or Node.

It occurs to me I'm looking more like 10 years into the future here than 15. Oh well. Close enough. I'm also well aware I'm just talking out my ass; I completely expect to look back and laugh at this in 10 to 15 years. Oh well again.