Well now you've got me thinking. That has to be very feasible for any content you own the file for, but I wonder how steaming services currently handle subtitles. Are they embedded into the video stream or are they their own channel? Could a third party pull them out and pass them along to a separate device like that?
Definitely not embedded into the video stream, at least for Netflix or Youtube. There's actually extensions that take the subtitles and manipulate them (even put them outside of the video stream) so you can use them for various language-learning purposes. Look at stuff like Language Reactor (https://www.languagereactor.com/), previously Language Learning with Netflix. There's specific browser extensions like subs2srs and yomichan (https://animecards.site/yomichansetup/) which are very common in Japanese language learning communities for people to use on subtitle lines to get direct translation/dictionary entries of certain words and be able to send them directly to Anki (SRS flashcard software).
Does anyone with more experience have any idea?