The Steam language setting determines which language the Steam client application displays, as well as the language which games are presented in:
- log into your Steam account;
- click the Steam menu in the top menu bar (this is the first option on the left at the top of the window);
- go to the Settings option;
- go to the Interface tab;
- select the language you wish to use from the drop-down menu;
- click OK and then select the option to Restart Steam.
Once Steam restarts, you will be able to use Steam and your games in the language which you selected.
If you have games installed that support the language you have selected, files for that language may need to be downloaded before you can play.
Source of the information and additional details can be found here
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