Access other languages of localized strings then current user language

[choncha]

Hi.

I'm having trouble with an application. It should be possible to choose a language in the app. This is necessary because it is a kind of guide. People should borrow a device with the app started. They choose a language and use the app. Afterwards the device is given to some other person. This person is able to choose another language.
The built in localization methods doesn't seem to cope with it. When using NSLocalizedString I can't choose a language by myself because this method only referrs to the default language of the phone. Is there any other way to access the other language string-files or to pretend that another language is the current default language??

Thanks
Björn