Apple has announced that the next version of OS X, 10.6 Snow Leopard, will be available in September, and posted its tech specs. As far as I can tell from what has been released, 10.6 will have no new language localizations (I would have expected perhaps at least Arabic) and the only expansion in language input capability may be Chinese handwriting for machines with a multitouch trackpad.
Under Key Technologies the specs say "Unicode 4". Hopefully this is a typo, since Unicode 5 has been available since 2007.
The Accessiblity page indicates that VoiceOver will be able to do all 18 system languages, but the extra voices will have to be purchased separately.
PS (6/17): I see the specs have been updated to say Unicode 5.1.