I think it's more a matter of "why would they have their system language set to X if they speak Y? If they want Y, they should just set their system language to Y!"
It's the idea that the user has a preference for something, and it applies always and everywhere, even when it's not applicable.
It should be absolutely clear, when i speak English and German, do not auto-translate any video title in those languages to the other. You wouldn't believe how bad the translations are, and how unwanted by me (the user). Worse when you speak a third or fourth language, and tend to watch videos. It gets messy.
It was more of an example in how they pick up on _some_ signal about a users language preference and then arrogantly assume they're correct in their decision, and that it's the user's fault if they assumed wrong.
This is actually something that foreigners working at Big Tech US companies should be able to understand very well, because English as a system language is often how software developers set things up for themselves regardless of their native language.
But they don't make those decisions. It's a UX thing, which means that in practice whoever is in charge of "driving up the numbers" is going to be making the decision; the engineers just get to cuss while implementing it.
Which has been baffling to me considering how many foreigners work at these companies.