There is a non-profit organisation that is providing tools to users to translate English content to other languages: https://translationcommons.org/academy/tools
Even the single language captions are mostly left to software and get it wrong often enough to be useless; youtube used to have a "community captioning" program, dont know what happened to that. At least on tiktok there's an etiquette of captioning the video yourself so it can be enjoyed on mute.
When I pirate a movie I can lookup corresponding .srt files to add captions in any language.
Maybe a browser extension that allows people to share caption tracks or alternate audio streams tagged to a video hash or url.
Maybe you could write a browser extension that injects your curated subtitle sources into video track tags?
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The short answer is that you require permission to do it legally.
There's a long tradition of "fansub" media, though, for things like anime. It circulated on VHS long before youtube.
> trustworthy
A more general problem with translations: now you've got to trust the translator. There's lots of room for distortion and loss or change of nuance, whether deliberate or accidental.