I liked the spectacle of Avatar, but never really found the story all that extraordinary. I found subsequent rewatches rather boring if I'm honest.
I hold BB and TDK as the best Batman movies and two of the best CBMs in general, but yeah the third act of BB was pretty bad. Jarringly bad in my opinion. It's a good thing the first 2/3 of the film was really good.
I think you answered your own question... the aliens have their own translators, and humans are hearing them through those translators (like Star Trek).
In AoS, they do not understand the aliens when said aliens are not speaking in English (which is a lot of times in that show).