Since iOS 4.0, this simply doesn't seem to be possible anymore. As of 2015, the documentation has been updated and now only states that Launch Images can be localized (but does not mention App Icons anymore).
The fact that this isn't working has been confirmed by an an Apple employee on the devforums (login required) in 2010: https://devforums.apple.com/message/330960
Bugreports have been submitted for several years, but nothing has changed. I suppose Apple wants you to use the same icon for your app all over the world – or create separate apps for specific markets.