Currently the audio/subtitle setting set by default for all content is defined on the server end. I have family who’s native languages are non-English (Swedish and Spanish) and I personally prefer my subtitles in English but some of my family members need them in Spanish and Swedish respectively. It would be great if there was a setting in the client that tells the server which subtitle language to use by default instead of having to change it for each video watched (which doesn’t work yet for shared content but will eventually I assume). Hope this makes sense 
I agree.
I reported this on Lighthouse a while ago: https://plexapp.lighthouseapp.com/projects/14382/tickets/1297-client-side-subtitle-language
The choice of language should be per user, not per server.
Early 2021 clean-up: implemented
