When you launch the game app for the first time, or start the game in your browser for the first time, it will automatically start in the language of the device on which it is used.
The supported languages are Spanish, French, Italian and English.
If you start the game on a device set to an unsupported language, it will launch in English.
If you want to change the language, open the Settings panel by clicking the top right button (the gear icon). In the next menu, scroll down to Language and choose which language you would like to use!
Then close that menu. The game is now in the language you chose!