If I recall the timeline in my Behind the Magic CDs, this also says that the movies happened before ROTJ.
If you want to know why they let the Ewoks talk english (basic), its very simple. They didn't want to bring in a narrator like that had in the first movie.
If you need an 'in-movie' explanation...how about this: Cindel and her family didn't speak basic (though we heard it). Wicket gains a rudimentary idea of the language.
In ROTJ, the rebels did speak basic.