To change your language settings:
Tap your username at the top left corner to open your profile.
In your profile, tap the Settings gear at the top right corner.
The second option in the Settings menu is Language.
You can adjust two settings here:
Menu Language: changes the language of the app interface.
Game Dictionaries: choose which dictionary (or dictionaries) you’d like to use while playing.
You can select multiple dictionaries and set one as your default, or simply choose a single language to play in.