String translation. This works differently to Drupal standard localization system: The strings will be translated from the default language (which may not be English), so changing the default language may cause all these translations to be broken.
Wouldnt the better system be that original Drupal and contrib English text strings are "neutral" and then users could add English (copy all strings from neutral, as it is already English) or Spanish (import po file or translate yourself) and switch between default languages as much as they like and all strings would be translated from "neutral"?
This system would make internationalization more consistent - by default everything would be "Language neutral" - strings, nodes, menus etc.
Is there discussion going on somewhere about this issue?