You did not specify what kind of subtitles you want to download.

If you want regular youtube subtitles in .SRT format (the most widely supported by players) then you would use (assuming you need English subs):

youtube-dl --convert-subs srt --write-sub -sub-lang en <YOUTUBE_VIDEO_ID>

If you want auto-generated youtube subtitles then you need to use instead:

youtube-dl --convert-subs srt --write-auto-sub --sub-lang en <YOUTUBE_VIDEO_ID>
#                             ^^^^^^^^^^^^^^^^

To list available subtitle languages use:

youtube-dl --list-subs <YOUTUBE_VIDEO_ID>

Once you have identified the languages you want you can request multiple subs with e.g.:

youtube-dl --convert-subs srt --write-sub --sub-lang="en,de,es,zh,zh-CN,zh-TW,zh-HK,zh-Hans,zh-Hant" <YOUTUBE_VIDEO_ID>"