Does not seems to work completely.
Video title is correcly not-translated on the channel page, however when I click to see it, then everything is translated.

See here, for instance : https://www.youtube.com/watch?v=AeJ9q45PfD0
My youtube is setup in french, I see a french version of title + description.
In private nav, unconnected, everything goes back to original english version.