I tried [userDefaults setObject:@"en" forKey:@"AppleLocale"] in the +initialize class method of my app delegate which as far as I know is about the earliest moment you can do something when your app is launching, but apart from the ugly side-effect of having this value written to the preferences plist file, even at this time it's obviously too late: all the formatter objects in IB still use the German locale. Only after a restart of the app (which now reads the respective entry in the preferences plist) the setting takes effect.