> Or it's just as dumb as this was on some product managers "designed and implemented" line to get promoted?

I'd suspect it's something banal, such as: $goal --> translate by default --> enough users click through by mistake (AB test shows user interest) --> more preroll ads shown to users (AB test shows business value) --> promotion

Whether the $goal was {accessibility, show off translations, UX improvement} is quite irrelevant for a business that optimizes for revenue from ads.