Here are the new language features I've seen so far in Snow Leopard 10.6.0:
1) Chinese handwriting input is possible for laptops with Multi-Touch trackpads when using Apple Chinese IM's.
2) The Chinese Pinyin input method is improved and some new Heiti and Hiragino CJK fonts are provided.
3) Polish is included the list of built-in spellcheckers, and you can now add .dic/.aff format dictionaries from other sources (e.g. OpenOffice ). Also you can set the system wide spell checking language without changing the system language itself.
4) The ability to set the keyboard layout as the same for all docs or different for each doc, present in Tiger but omitted in Leopard, has been restored.
5) Bidirectional text (e.g. English/Hebrew) now has system-wide settings for split-cursor, and for RTL, LTR, and Default text directions. (But it looks like RTL problems in iWork/iWeb have not been addressed).
6) VoiceOver is now able to handle all 18 system languages (but only English is provided by Apple).
7) Unicode is upgraded to the latest version (5.1 of 4/4/2008)
8) A "US International - PC" keyboard layout is included, which will be welcomed by Switchers accustomed to using this for W. European languages. (Another has been available on the Web for some time, however.)
9) Hitting Space while holding down Apple/Command will produce a list of active keyboard layouts in the center of the screen, which can be selected via the mouse or the up/down arrows.
10.6 has no new localizations for OS X or any new languages for reading and input.
Also it appears that File > Get Info no longer has a Languages tab, which makes it hard to run an app in a language other than that of the OS. A workaround may be here.