@marcos For a sitewide change, go to your Admin Control Panel, then go into Languages, select from there. If you want to change the language on a per user basis, then click your profile picture in the top right, then your username, then in the top right you will see Settings, scroll to the bottom, and you can pick there instead.
If the language you want is not in the app, have a look at Transifex, where you can help translate if your chosen language isn't available.
