Er, there is - pick a language and make that the standard....
.... which of course is the sticking point of this thread.
Fine for INTERNATIONAL, but not for national flights.
Why should a language that is not native to country X be mandated to everyone who wants to fly in country X, who does NOT want to fly internationally?
If you only fly in France, you should not have to speak English to be allowed to fly.
If you only fly in Russia, ditto.
That's why it's not simple.