Our forum member MadSheep! prepared this step by step manual - How to install Windows 7 language pack.
Download and extract the language pack that you want after the extraction you will have the lp.cab file
Go to Start> Control panel> Clock, Language, and Region> Change display language
Press the Install/uninstall languages... Button next the Install display languages Button and then the Browse computer or network Button
Press the Browse button and locate the lp.cab, Select your language and press the Next Button
Accept the license terms and press next to Install