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There are a number of language options for programming on OS X, but the Cocoa frameworks are written with Objective-C in mind. If you want to get the full power and flexibility from Cocoa, then you should use Objective-C.
Bridges or interfaces to a number of other languages exist, however, and provide varying capabilities. If you are more familiar with one of these languages, or need to integrate with other code written in them, then you may prefer to use them.