OS X 10.5 Leopard was made available for preorder today, and Apple also published its tech specs and new features.
These indicate that 3 new localizations -- Russian, Polish, and Portuguese (Portugal) -- have been added to the previous 15. The full list is now English, Japanese, French, German, Spanish, Italian, Dutch, Swedish, Danish, Norwegian, Finnish, Traditional Chinese, SimplifiedChinese, Korean, Brazilian Portuguese, Portuguese (Portugal), Russian, Polish.
New languages mentioned as supported via keyboards/fonts are Tibetan, Kazakh, Uyghur, Kurdish, and Jawi (Malay in Arabic script).
New spell checkers are Russian and Danish.
Dictionary.app now includes Japanese (with a translation facility) in addition to English.
Here is a list of the language-related items from Apple's page describing Leopard's features.