[dev_slash_null]

Let me clarify what I'm claiming here.

In many counties, users have their OS/Browser settings set to English only, when they actually also speak other languages (or in certain countries, exclusively other languages). There's also a bunch of complexity in regards to spoken/written language comprehension, but I won't get into that here.

There is much more engagement when Google presents content in languages people actually know (or, rather, languages Google thinks they know), regardless of what their Accept-Language header is.

Unfortunately, I cannot present you with hard stats, as I no longer work there. I'm sorry that's unsatisfactory, and I'm not asking you to blindly trust the word of a stranger. Just wanted to offer up some insight into why the system behaves as it does sometimes.