Reasons it's useful: * If you've got both Native & Target Language subtitles, you can see a natural translation if you're struggling to understand something * If there isn't a Native translation, then you can machine-translate one - especially useful early on to catch common idioms/etc that aren't just the sum of each individual word. * Jellyfin also supports eBooks, although its reader isn't great - but if someone has already built their library, it would be nice to be able to re-use it somehow.
I would be very interested in seeing that particular feature expand, but I don't imagine it's at all simple!
Tangentially related, but I could see some desire for Calibre support as well, somehow. Calibre was very much designed to be completely stand-alone and it doesn't really support other apps trying to read its database, but it is possible.
I'd also really like some language-specific features, like separable-verb handling for German (see this comment: https://news.ycombinator.com/item?id=38915786) - it's relatively important and lacking support really limits the usefulness of vocab tools. It would also be a nightmare to handle for subtitles, since it's not always clear where a sentence ends, but such is life - subtitles are sadly not aimed at language leaners. For books and not-terrible Podcast transcripts, though, it wouldn't be so bad.