Technically they're frameworks, not languages. Just nitpicking.
And for those who see us going further down the rabbit hole here, a framework is a series of "pre-packaged" programming functions (also known as APIs, libraries, or objects) that programmers can leverage to avoid re-inventing the wheel, as well as ensuring consistent behavior across different applications. Window manipulations, UI elements such as buttons, menus, scrollbars, etc, as well as functions such as audio or video playback, text editing, high-level network functions such as HTTP, etc. are all available in frameworks, saving developers the trouble of having to write their own code.
Carbon and Cocoa are two different frameworks that MacOS X supports - Carbon is very similar to MacOS 9's API, and was developed to provide a "bridge" for MacOS X to support apps originally written for MacOS 9 without requiring developers to rewriter their apps from scratch. Cocoa is a completely different beast, far more powerful by most accounts, but generally requires developers to completely rewrite their code if porting a legacy app; however, it is the recommended API for MacOS X development going forward - in fact, Apple has announced that there not be a 64-bit version of the Carbon framework, and no new API features will be added to Carbon.
In this case, Finder is one of the few (if any) remaining Carbon apps that ship with MacOS X, so while moving it to Cocoa seems like a primarily under-the-hood feature, it means that it's now a 64-bit application (there's no 64-bit UI support in Carbon) and can benefit from new framework enhancements that won't be added to Carbon.