If a user doesn't know about it, he probably has the browser set to his native language, which resolves the issue before it's an issue.

I might be outlier, but I like my software interfaces to be in English. Keeps them all the same ('save all', etc)

But I would rather consume content of a website in my native language. While I'm not complaining if it it will serve me English.

Firefox has two separate settings for "Choose the languages used to display menus, messages, and notifications from Firefox." and "Choose your preferred language for displaying pages" so your use case should be covered by not second-guessing the user settings.

Having your software interface in English also makes it easier to find help when you’re confused by something.

Maybe true in the West. Not totally true in a lot of countries where the OS is almost always in English despite the fact that user may not be able to comprehend English (they can read lone word just fine to operate menu and such)

By default browsers get that from the OS, and that was typically set on first boot. It's a very reliable signal.

This seems well within Google's (especially) ability to fix.

They seem to rely on similar settings for knowing whether to bring up a "This page is in Polish. Do you want to translate it?" popup.

What do you mean? I don't think we're talking about language preference, simply about...language of the browser itself.

There's a setting for language preference that is set in the browser, and transmitted in every request header. This is the thing that we're talking about.

Right, I have no idea how it works(and I guess like was said, 99% of people don't), but as a user I'd reasonably expect that if I have my browser in English, my operating system in English, then I want to see websites in English.

We don't need to know how it works, it asks the web pages to serve content in that language, it's made explicitly for this use case. It should be the first choice. Second choice could be browser language.