Users don't configure it (and quite possibly don't know how) so many websites ignore it.

Many websites ignore it so it isn't that useful to learn how to configure it.

#2 is the site language cookie from a previous visit. If that fails, #3 is the language the browser requests. If all those fail, English, because it is the most common language the company uses internally.

In our user testing, we run into a lot of situations where the computer's primary user, and the person who set it up, are a child, who speaks English. But our site is more geared toward their parents, who often will speak the language of their homeland. For this reason, I make it stupidly easy to change languages, and we have a policy that ALL content has to be translated into English, Spanish, and Chinese, which covers 99.8% of our customers.

You don't find out these sorts of things through modern "telemetry" gathering and such current methods of testing. You find these things out by going into the field and watching your customers use your products. But interacting with wetware is beneath people in the SV bubble.

While not a perfection solution I feel if the user is able to understand the language of the OS they are using then giving them a website in the same language is much better than picking one based on an IP location.

I totally agree the browser language options are hidden with a pretty crappy UI that no normal user would find and use easily.

I'm currently in Switzerland. Different regions speak German, French and Italian (among others).

If you just use "this IP is in Switzerland" to try and guess the best language, you will definitely get it wrong. 25% of .ch pages I have never visited before start out in French, most others in German.

No rhyme or reason.