Here someone asked if it's possible to contribute subtitles to a YouTube video. I know that it's possible because I contributed Italian subtitles for this video (because there is an option to do that).
The following screenshot shows the option:
Clicking that brings me to this page.
Why can't I find that option on every video?
Is it something that uploaders must explicitly enable?
Are there some legal issues?
Only some channels allow users to contribute subtitles.
From Contribute subtitles and closed captions - Youtube Help
Some channels let you contribute subtitles and closed captions to their videos. You can submit content for an entire video or just add what you know — every little bit helps make videos more accessible to a larger audience.
If the video's creator accepts your submission, they'll be viewable on the video by clicking on the [CC] icon in the player. Approved content is owned by the video owner, but your YouTube username and public profile may be visible to the video uploader and other video viewers.
Before you start: Make sure the video you want to add captions or subtitles to is part of a participating channel.
