If I might say so, and I guess others would disagree, most JAV isn't much better, that is more exciting or erotic, if you know exactly what they are saying. I think only in the case of some of the drama leaning vids it makes it worth watching the dialogue instead of zipping to the naughty bits, but it's not really an improvement to know the actress is saying "give me your cum please" compared to "shoot on my face" and all the variations on the theme.First off, I envy your Japanese fluency. Must be nice to be able to actually understand your favorite movies. After so many years of watching anime and JAV, I'm slowly gaining some spoken fluency, but I'm still at baby-talk levels. So I have to rely on the incredibly rare subtitles I have, many of which I translated myself from Chinese subtitles.
Anyhow, it's not like I haven't added features that don't personally benefit me before. I think it would be worthwhile to make JAV multilingual. It's more about my limited available time and the long list of features I'd like to add.
Being able to read the covers, and use DMM and the like is definitely useful. However, this is all fantasy, so being able to imagine the relationships between the actors in your mind without being forced by the actual story and dialogue is a thing too. More than half the time I turn off the sound so I don't have to hear that actor A is the father-in-law or brother, and that actress A is the mother, sister-in-law or the wife. Depending on what I would prefer, I just use my imagination to create the relationship instead. Best of both worlds, I guess?
Regarding Anime subs, I am under the impression there are a LOT of them available. Crunchyroll perhaps? I am sure there are how to learn Japanese through anime websites out there. If you aren't living there and being immersed and forced to learn it through necessity, it can be pretty tough. Good luck with your journey to learn Japanese!