Summarize:

If you are using Windows 7 Ultimate or Enterprise, you can install language packs that translate most of the Windows interface. These are only available for Ultimate and Enterprise. If you are using Starter, Basic, or Home, you can install Language Interface Packs (LIPs). These translate parts of the interface, and require a base language installed. See the next section for details. You can open the Control Panel from the Start menu. This will allow you to quickly access any Control Panel option. You can use the Windows Update tool to download any of the available language packs. If the link is not there, click "Check for updates". After selecting the language, click OK. Install updates. You may be prompted by UAC to continue, and you may be asked to input the administrator password. It may take a few minutes for the language pack to download. Select the Keyboards and Language tab. All of your installed languages will be listed here. Apply and then Log off now to log out. Your changes will be applied when you log back into Windows. Some programs may not display your new language until you change your system locale settings to match that region. Open the Control Panel and select "Region and Language". Click the Administrative tab and click Change system locale. Select the language you've just installed and click OK. You'll be prompted to restart your computer.
Open the Control Panel. Click the "View by" menu and select "Large icons" or "Small icons". Select Windows Update. Click the "# optional updates are available" link. Check the box for the language you want to download. Click the . Return to the Control Panel and select "Region and Language". Select the language you just installed from the "Choose a display language" drop-down menu. Click . Change your system locale if the language is not displaying in some programs.