Each time your app returns to the foreground (not when it becomes active), get the current locale. Compare the latest locale to the previous locale. If the new and previous locale are different enough for you, post the notification.
Example - going from Spain to Mexico should change the locale from es_ES
to es_MX
.
Edit:
To post the notification you can do:
[[NSNotificationCenter defaultCenter] postNotificationName:NSCurrentLocaleDidChangeNotification object:nil];