The application host determines the language, you are right. While I completely understand the desire to offer more translations of your product, I don't think it is a great idea to go beyond what Address Book offers, or do hacks that would cause the UI to display in both one language, and another.
Mostly I say this from the standpoint that an app that displays in two languages at the same time might be confusing, and slow down the user as they sort of mentally 'context-switch' between the languages while using the app.
I'd keep the translated files, and perhaps swap out the English resource with the translated resources to test them on your machine... but otherwise leave them alone. Maybe petition Apple to translate Address Book into more languages for your users.