YouTube has had machine generation of subtitles and then machine translation of those subtitles for a decade. All it needs after that is text-to-speech which is not hard. Translation is much quicker than the necessary transcoding of the video stream.

The reasons it's not there on YouTube are manifold, and include product (many people hate dubbing and want subtitles) and legal (you are making another audio track with the copyrighted sounds, you are wholesale using someone else's video to make a new video, the owner of the original content may not like the auto generated new sound track, etc.)