As far as I can tell, it's available on desktop, but neither in mobile browsers nor the app. The video language being the same or different from the UI language didn't seem to make a difference.
My current working theory is that they couldn't figure out a way to fit a two-level menu on a phone screen, so they just cut that feature...
I just tested, and yes on desktop there is an 'Auto-translate' option, even for videos with 'auto-generated captions'. However on mobile (YouTube app on Android, Firefox, Chrome) and my TV the 'Auto-translate' option is missing, so I can only view the generated subtitles in the original language. I mostly watch on mobile or TV, so :-/
My current working theory is that they couldn't figure out a way to fit a two-level menu on a phone screen, so they just cut that feature...