Actually, Cocoa and Carbon are not languages they are APIs. You access these APIs through some programming language. In the case of Cocoa, you access it with Objective-C, which is a superset of C. Objective-C is just like C, only with additions for handling object-oriented programming.
Most APIs are accessible through more than one language. There are several bridges available that let you write Cocoa programs in other languages besides Objective-C. Apple includes a Java bridge in XCode, and I've seen some for Ruby and I think Python, too. But Objective-C is sort of the "native" language of Cocoa (for lack of a better term).
I hope this clears it up a bit. I especially hope I haven't made it even more confusing. The relationship between Xcode, Cocoa and Objective-C is often misunderstood.