Can I switch to Arabic or dark mode before I sign in?

Yes. The first screen of the app, with the top-rated coaches, has a small switch at the top: tap ع | EN to change the language and the sun or moon to change between light and dark. Sign in, Create account and the password screens have it too. The app starts in your phone's language and theme, and what you pick there follows you into your account. You can change both later in Settings.