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Posted above as well but mpv user script that works offline: https://github.com/oltodosel/interSubs

Ognjen (guy I worked with to make this extension) previously made something similar for offline media (https://github.com/oaprograms/lingo-player). It's neglected now though and probably suffered bitrot.

We could use the code from our extension to support offline media.. but Chrome doesn't support MP4 playback IIRC. It's possible to make an Electron/NW.js app, but the vlc plugin isn't working with latest versions, last time we checked. Appreciate any info.

You could use the .ass format for fancy subtitles. Players like vlc, mpc, mpv support it. That would give you the subtitles in both languages plus the highlight of the words as they are spoken but clicking on the word to get a definition would need to be built into the player.