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Several years with help of volunteers. There's also a bunch of people writing subtitles for the recorded talks; live streams are automatically subtitles nowadays. Some years ago we would even do manual subtitles for live talks.

It's nice that they do it, but note that adding multiple audio streams to one video is quite easy to do. (Translating the talk in the first place, not so much.)

You can join the subtitling efforts here: https://c3subtitles.de/