I second the mpv-based solution, as suggested by st1ck. If you prefer vlc, it now (as of the beta 4.0.0)[0] also supports dual/secondary subtitles. Making it work seems a bit fiddly: first you need to turn them on (under Tools > Preferences > Subtitles/OSD > Dual Subtitles (at the very bottom) > Align — change to anything but unset, and possibly also adjust offset). Then, when playing a video, to select them for that video, you need to "Toggle secondary subtitle control" with Ctrl+Shift+V (this means that the normal subtitle control shortcuts like "v", "Alt+v" etc. now apply to the secondary subtitles) and press "v" the right number of times to switch to the subtitles that you want.

(Obviously vlc and mpv will only work for DRM-free videos, e.g. from youtube or a DVD.)

If you were asking specifically about Netflix subtitles, there used to be an open source NflxMultiSubs extension[1] for both Firefox and Chromium, but it was broken by Netflix introducing changes to its video player and discontinued. There is an active, open-source dual-captions[2] extension, but it's for Chrome only. (However, since it's open source adapting it it for Firefox should be straightforward.) Finally, as an alternative approach, you could try a Firefox addon which allows loading arbitrary subtitles in the SRT format to netflix,[3] and which might perhaps allow you to have both netflix's subtitles and your own SRT ones at the same time.

[0] https://github.com/videolan/vlc/blob/master/NEWS#L30

[1] https://github.com/dannvix/NflxMultiSubs/

[2] https://github.com/mikesteele/dual-captions

[3] https://addons.mozilla.org/en-US/firefox/addon/netflix-srt-s...

NflxMultiSubs is still working. I wrote a patch to fix it, and took over maintence on Chrome.

https://chrome.google.com/webstore/detail/nflxmultisubs-netf...

> NflxMultiSubs is still working. I wrote a patch to fix it, and took over maintence on Chrome.

That's great! It's a shame that it's (apparently) no longer open source, though obviously given the MIT license you're allowed to stop disclosing the source.

Thanks for the shoutout! :) - mike

    mpv --sid=auto --secondary-sid=auto --sub-pos=95  --slang de,en movie.mkv

Same works for youtube URLs. If it doesn't work, changing "de,en" to "en,de" may help.

mpv is so great. It was an immense downgrade to start watching movies on my ps4 with media player, netflix, plex etc. They(mostly media player) have issues on the very basics like pausing and playing video, subtitles, continue watching etc.

With a computer running mpv and plugged into the TV everything just works and I can trivially fix common issues like subtitle/audio sync with my phone as the remote.

On-topic: mpv seems to have a similar language learning extension to OP https://github.com/oltodosel/interSubs

I've made my own mpv extension for Japanese subtitle analysis too. It can also create Anki cards automatically (with screenshot and audio).

Having a scriptable media player is really something else.

This exists, albeit only for YouTube rather than Netflix: CaptionPop https://www.captionpop.com/

Creator here.. there's netflixmultisubs extension which is more minimal, which we also maintain now. There used to be releases for firefox, but we haven't been publishing them, just becuase we're not familiar with the process and lack time. LLN can be used in a minimal, non-instrusive way however, and has a pretty good algorithm for aligning the start and finish times of different subs tracks. netflixmultisubs shows the two tracks with their independant timings.