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
Step 2
Go to Start> Control panel> Clock, Language, and Region> Change display language
Step 3
Press the Install/uninstall languages... Button next the Install display languages Button and then the Browse computer or network Button
Step 4
Press the Browse button and locate the lp.cab, Select your language and press the Next Button
Step 5
Accept the license terms and press next to Install
Source: KezNews
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