youtube-dl legendas download

--write-sub                      Write subtitle file
--write-auto-sub                 Write automatic subtitle file (YouTube only)
--all-subs                       Download all the available subtitles of the video
--list-subs                      List all available subtitles for the video
--sub-format FORMAT              Subtitle format, accepts formats preference, for example: "srt" or "ass/srt/best"
--sub-lang LANGS                 Languages of the subtitles to download (optional) separated by commas, use IETF language tags like 'en,pt'

Baixar somente a legenda:

youtube-dl --all-subs --skip-download  https://www.youtube.com/watch?v=aaaaa