I've been reading up on GUI frameworks including WPF, GTK and Cocoa (UIKit). I don't really do anything related to Windows (I'm a Mac and Linux guy) or .NET, but I'd like to be able to throw together GUIs for various operating systems.

We are in the enviable position now of having high level scripting languages that work with all of the major GUI toolkits. If you are doing Linux GUI programming, you could use GTK in C, but why not just use PyGTK (or PyQt). Similarly, for Java, one can use JRuby. For Mac, there's MacRuby. And on .NET, there's IronRuby. This is all fine and good, and if you are building a serious project, there are tradeoffs that you might encounter when deciding whether to, say, build a WPF app in C# or in IronRuby, or whether you are going to use PyGTK or not.

The subjective question I have is: what about learning those frameworks? Are there strong reasons why one should or should not learn something like WPF or Cocoa in a language one is familiar with rather than having to learn a new language as well?

I'm not saying you should never learn the language. If you are building Windows applications and you don't know C#, that might be a bit of a problem. But do you think it is okay to learn the framework first? This is both a general question and a specific question. I've used some Cocoa classes from Ruby and Python using things like PyObjC and there always seems to be an impedance mismatch because of the way Objective C libraries get built.

Experiences and strong opinions welcome!