Currently, the UIs (primarily select inputs) that display the list of available internationalization locales appear to sort by the numeric unique id of the localization when it is added in settings.
This means that the last locale added is always the last in the list, even when from a user experience perspective this might be the locale you are selecting the most often since it was the last to be added and needs content applied to it.
Ideally an admin should be able to choose the order in which locales are displayed to content managers.