As far as I know you can't change the language of windows . I.e. if you buy a Spanish version, all menus are going to be in Spanish no matter what regional settings you choose. For example I've got an English version of XP, but my region and input settings are set to Danish. I still get all menus, help files, etc in English (the way I like it).
So again as far as I know (I might be wrong) you'll need an English version of Vista in order to get English. Although an English upgrade disc might work as well.