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Internationalization is the process of designing software to accommodate multiple locales and creating user-interface content that is easy to localize. The Cocoa frameworks in Mac OS X do much of the internationalization work of developers for them. Mac OS X services in Carbon, Cocoa, and Core Foundation help developers internationalize their software and manage locales in a consistent manner.

A guided introduction to internationalizing software for Mac OS X.   Essential information for developers designing software for multiple locales.  
Internationalization Topics
C APIs that ready applications for other locales.   Object-oriented APIs that ready applications for other locales.   An international standard and the native character encoding for Mac OS X.