Second, that's not an app development platform. Gnome has C (and it's own unholy stack on top), KDE has C++.
You can code for OS X/iOS in all kinds of languages too, if you mean at the OS and not at the app development level.
If "platform" means "app development platform" and that means "full-featured toolkit for the development of GUI applications that run in an integrated environment", then I would agree both that Gnome and KDE fit that bill and that they have limited language support (in my experience the only language worth using with those toolkits other than the respective C and C++ is Python).
And you can't do this in OSX or iOS?
> The average Unix desktop or server is probably running code written in 5-10 languages, more or less transparently to the user
The average iPhone is probably running code written in just as many (if not more) languages - C, C++, Obj-C, Swift, and Javascript are a given before you even look at third party stuff. And, modulo sandboxing, you can do all the Unix stuff too.
I actually don't have any opinion on how many languages have 'first-class' support on iOS. My understanding is that it's somewhat more complicated to create natural-seeming bindings from other languages to Objective-C APIs than it would be for C APIs, and that there are restrictions in the type of applications you can distribute using other languages (no downloadable code outside the App Store). Whether that makes those languages not 'first-class' on the platform is up to interpretation.
But the person I was responding to was specifically asking if any platform had first-class support for more than 1-2 languages. I don't know how to determine 'first-class' as a matter of principle, but I assumed that the multiple decades of polyglot software development on Unix environments would count. (OS X would count there, by the way.)