Whenever you start a game by clicking on "New Game", you'll see a screen in which you'll be able to choose the language and the type of opponent you wish to play against.
To select or change the language (before you start playing), you must choose the flag that represents the language you wish to play in. We recommend you pay special attention when choosing the language because, once the game starts, you won't be able to change it.
Bear in mind that you may be invited to play in a different language from the one you usually play in. When this happens, we recommend you decline the invitation, since this won't count as a lost game.
If you choose the language but are still served a question in another language, please report it for "Wrong language".
For more information or to know other users' opinions, visit our Official Forum and check this post.