Motrix has no account and no sign-up. It sends no notifications. Your games, your streak and your statistics are stored on your phone and nowhere else.
The fifteen dictionaries are inside the app. Motrix plays in airplane mode, on the subway or on holiday. The word of the day is computed on the phone from the date and the language: no server sends it.
The network is used for two things: the ads, served by Google AdMob, and the purchase that removes them, through Google Play. In Europe, a consent form asks your choice before any ad. If you refuse, Motrix requests no ads. You can change your choice in Settings, under Ad privacy options.
The details are in the privacy policy.