When I released version 2.0 of my app Photo Timeline, some users reported updating problems. I finally found out what caused the problem and submited a new build to the Windows Phone store.
In my app I had three localizations: en-US (default), en and cs. I do not exactly remember why I had English twice as localization. I decided to remove the en localization thinking that nothing will break and that was a mistake.
Once you add a localization to your app, never remove it! If you do so, all the users who use your app with that localization will have update problems.