Anonymous
I have never “logged into” youtube and do not foresee doing so in the future… so I cannot personally test this “bright idea”, but
While not logged in, I can request auto-subtitles and they are presented to me in English.
Possibly this detail is based on my browser’s user-agent string (or other http request header).
Possibly a logged-in user can choose different, specific, locales for the UI (and subtitles).
In addition to the overlaid subtitles… the text along with (or without) timestamps is displayed to a scrollbox at the right side of the page and can be captured (copypasted). Harvesting these, for various languages, might provide a head start for your proposed project.