Check out the new Windows 7 language download page. On this page, you can find the 95 language versions available for Windows 7 or Windows Vista and install them as extra languages on your system.
For example, if you have Windows in Spanish, you can add Basque, Catalan, Galician and Quechua for free and have menus, dialogs and text displayed in any of these languages. With an Ultimate or Enterprise edition of Windows, you can add just about any language to your original language version.
You’ll find similar overviews for Office and Visual Studio here:
…and you can of course find the translations used in these languages on the Microsoft Language Portal Search page.