As a site admin you can change the default language for the entire organization from Settings (Side-pane) --> Preferences --> Click on the Language selection drop-down and select the language --> SAVE.




As a user, you can set your own preferred language by Clicking on Edit Profile icon below your name --> Preferences Tab --> Select Language --> SAVE




Please Note: Changing default language as a site admin (from the Settings menu) will reset the users' preferred languages.