With Ultimate, you can download the language packs for free.
Just open Windows Update, choose "view available updates",
and check the language pack you want.

Afterwards, from Control Panel (categories view), just choose
"Change display language" under "Clock, Language and Region".
Pick your language, log off, log on, and done.

If you have another version of Vista other than those two,
such as Windows Vista Home Premium,
you'll need to purchase a "full version" of Windows
Vista in your desired language and perform a "clean install" or
install it on a separate partition or drive (dual-boot).