1. There are languages other than English & Spanish, like hundreds of them and it might be rare but why not have the option when we need it?That's not realistic.
For one, that situation is pretty rare. As a Spanish speaker with grandparents that only speak one language, I can count the number of times where this would have been useful in one hand.
Two, this would require playback of two audio channels going to two different audio sources, which in itself isn't impossible. But then you have to turn surround sound into mono sound, negotiate new contracts with content developers aka every studio and most importantly you'd have to get Apple to care about it.
Siri can't even get it right when I request music by Juanes or Enrique Iglesias yet they are going to support that? I'm just saying, don't get your hopes up.
2. Turning surround sound to mono can be fixed by using 2 AirPods. I guess it's possible with AirPlay 2.
3. The file might not be a movie at all but then again, I think this is negotiable with the studios and they might not even care since you're buying the movie in the first place.
4. What does this feature, if implemented have to do with Siri?
I like the idea so much I'm emailing Apple about it