[mavhc]

If only browsers sent some kind of message to the server to tell it what languages you'd prefer

[cbhl]

They do, and have since at least the mid 90s. But most users don't know how to set this preference, so Google assumes GeoIP is the One Source Of Truth(TM) for language (apart from hl= and being signed in).

[timClicks]

I don't think you had your sarcasm filter turned on there.