You can only change your languange setting on the game's login-screen. So either
1) you try to leave the game you are in and log out. The "Resign" button in teh game is on the top right, in the lobby the lowest entry in the bottom right is "Logout". Then you can simply select "English" on the log-in screen (top left), or
2) if you can't find out, you need to open Safari in private mode (from a google search, I don't use Mac and/or Safaro: Go to File > New Private Window from the menu bar (Another way to do this is to click Shift-Command-N using your keyboard shortcuts), go to "dominion.games", enter your login credentials, then click on "Kick" under the usual login button. Then go back to a normal window and again go to "dominion.games" and login, making sure that English is your selected language.