From Microsoft: How do I get additional display languages?
You can add display languages by installing additional language files. When you install language files, you can view wizards, dialog boxes, menus, Help topics, and other items in Windows in a different language.
There are two types of language files:
Windows 7 language packs: Windows 7 language packs provide a translated version of most of the user interface. Language packs require a license and are available only in Windows 7 Ultimate and Windows 7 Enterprise. Language packs are available through Windows Update as optional updates. They aren't installed automatically.
Windows 7 Language Interface Pack (LIP): Windows 7 LIPs provide a translated version of the most widely used areas of the user interface. LIPs are freely available to download. Because not all of the user interface is translated, LIPs require at least one parent language. The parts of the user interface that aren't translated into the LIP language are displayed in the parent language. When you download the LIP, you get the parent language requirements for that language. The parent language pack needs to be installed before the LIP can be installed. For more information, including a list of languages available for downloading, go to the Microsoft Local Language Program website.