Big tech has changed over the last bit to trying to tell us exactly what and how we should think - or maybe more precisely to see thought as “friction” and want to remove it all together. This is a very minor example in the scheme of things, but I see it everywhere now.

Not really, reliably detecting a user's preferred languages has been a persistent Hard Problem in tech since the start of the internet. Every proposed alternative solution ends up having vastly more false positives due to browsers/people incorrectly setting of default preferences so companies begrudgingly default to geographic heuristics knowing it is a terrible experience for an outlier group of people.

Why wouldn’t you just ask them, and particularly for media like this that has a native language, default to that? I don’t want software to think for me about what I want to see, if I want something different I’ll change it.

You're going down the first question of a deep rabbithole that eventually lands you to where every major tech company has landed.

Got it, "maximizing ad revenue " it is.

Yet this implementation that Youtube uses is in the bottom 1% of worst ones. Strange how roughly every other website/app does a better job at it.

It's probably Microservices(TM)... The black box responsible for rendering the HTML is some other black box to the UI or whatever. The UI offers you locale options, but the renderer that fetches video titles hasn't been configured to respect this, probably uses locale from geolocation and some overpaid genius said "always use machine translation if locale doesn't match video title language"...

The homepage of google.com is also localized. I remember noticing that even when requesting and getting the English locale, the tooltip for the doodle was still in my region's language... wahey!