Here's a link
How To Add and Enable Additional Languages in Windows though no windows vista information on how to add it.
for windows XP:
To install another language and keyboard layout in Windows XP, follow these steps:
1. In the Windows XP standard Start menu, click Start, and then click Control Panel.
In the Windows XP classic Start menu, click Start, click Settings, and then click Control Panel.
2. Double-click Regional and Language Options.
3. Click the Languages tab, and then click Details under "Text Services and Input Languages".
4. Click Add under "Installed Services", and then click the language you want to add and the keyboard layout you want to use for that language.
5. To configure the settings for the Language bar, click Language Bar under "Preferences".