localizing an app

Not necessarily a framework thing, if I understand you correctly you're talking about creating base functions in a class, then extending that class with extra functionality? If that's the case, it's a basic php feature that's available on all frameworks. In Codeigniter you can extend the basic CI_Controller and add functionality there.

Codeigniter is simply one of the tools you need to learn to be a successful developer. Always add more tools to your coding arsenal!