Can you say more about what you’d want in such a service? What language are you learning? What kind of videos do you watch? Where do you get them? Youtube?
If there was a collection of videos on you tube which had subtitles for the main video and an extra language then I would pay for it. Danish for me
Nice. I working on something that does exactly that for Korean videos on youtube. If you're interested in helping with feedback, I could repurpose it for Danish. If so, email me at jon@jonb.org.
Send me a link to try it for Korean and I'll give you my feedback. You can post it here on Hacker News