In order change your default system language, first click on your photo or avatar on the top right of the screen:
Click on "Settings":
Scroll down until "System Settings" and click on the pencil to edit the language:
Then, click on the 3 light dots (this interface is going to be improved):
Choose the language you want and hit save:
The platform should automatically be changed.
You also have the possibility to change the language on the app side. To do so, click on the hamburger menu:
Click on settings:
Click on the dropdown and select the language: