"you can change the display language on your computer so that you can view wizards, dialog boxes, menus, Help topics, and other items in Windows in a different language."
http://windowshelp.microsoft.com/Windows/en-US/Help/35a1b021-d96c-49a5-8d8f-5e9d64ab5ecc1033.mspx
What Exactly do the language packs do?

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GameFAQs
GameSpot
Giant Bomb
TechRepublic