This is a copy from Steam Support:
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The language of many games can be changed independently from the Steam client language.
1. Log into your Steam account
2. Right click on a game from the Steam game library and choose Properties.
3. Go to the Language tab
4. Select the language you wish to use from the drop-down menu
5. Click OK
If the game supports the language you have selected, files for that language may need to be downloaded before you can play.
If you are experiencing problems enabling a language, make sure your desired language is selected in both locations.
Note: Some languages may not be available for the game you wish to play. Check the Steam store page for the game to see which languages are offered.