There is no fallback mechanism in the current implementation. I have patches in my tree to enable a simple, "lang_country" -> "lang" -> "", fallback. However, we may want to consider a little more sophiscated scheme such as "user's locale" -> "default application locale" -> "none" type fallback. For example, The strres class is provided with "fr_CA" and the application locale is "en-US", then the fallback path will be fr_CA -> fr -> en_US -> en -> none
I had checked in a patch to perform the "fr_CA ->fr->none" part of the fallback. See bug http://bugzilla.mozilla.org/show_bug.cgi?id=8188 for details.
This is not a M8 show stopper. We need more discussion on how the mechanism shall work.
non show stopper-> M11