When Ewoks start using Queen's English, it's more than my suspension of disbelief can handle ...
No worse than English-speaking humans existing "a long time ago in a galaxy far, far away."
When the beginning scrawl's telling you "... this is just a fairy tale," that simple statement would seem to deflect all criticism, would it not? Then again, Gone With the Wind had one at its beginning too, which didn't shut up audiences, either.
But, surely, an actor representing a Human Being onscreen is expected by an audience to use the shorthand of Human Language, to be understood. If said actor comes onscreen with a green mask over his head, that has water-filled, bulbous plastic eyes, then reading subtitles becomes less of an odious chore. It might even be expected, by the audience - tolerated, certainly.
Teddybears shouldn't talk at all, in my view. They should have been given some gestures, grunt a fair amount, and dance around, with the least provocation. In light of the fart joke in Phantom Menace and all the belching in RETURN of the JEDI, it would've followed to have Ewoks throwing dung at the Stormtroopers, as a weapon. Or, at the very least, as a show of annoyance ... surely!