I know it is rude, but I do, sometimes, correct the grammar of native speakers who really ought to know better. With foreigners, I ask if they would like to be corrected if they make a mistake. When I speak foreign languages, I ask to be corrected if I make a mistake. I rarely make mistakes in my native languages, but appreciate correction if I do. But I won't dumb down to the level of the ignorant just to be polite.
I have learned to use local phrases like "store-boughten" but my tongue is visibly in my cheek when I use them.