The object in NSUserDefaults for @"AppleLanguages" should be a instance of NSArray not NSString.

[leavesster]

It seems that [[NSUserDefaults standardUserDefaults] setObject:@"fr" forKey:@"AppleLanguages"]; does not work.
[[NSUserDefaults standardUserDefaults] setObject:[NSArray arrayWithObjects:@"fr", nil] forKey:@"AppleLanguages"]; works fine.