Doesn't ad up
Many moons ago—over eight years' worth, in fact—I lamented Google's inability to use HTTP headers to detect language preferences. Things haven't changed much, if at all, since then.
This is brought home to me on the rare occasions that I get ads on YouTube, which is owned by Google. I watch YouTube via my browser, and I run AdGuard. This combination gives me ad-free viewing, but without having to pay Susan Wojcicki's Minitrue a single penny. WIN!
When ads do get through, I think that it might be due to Safari launching before AdGuard. No problem, I just close the browser and relaunch it. Ads nixed!
What irritates me is that the ads are in German or, worse, Swiss German. So, they're of no bloody use to me, even if I was interested in what they're peddling. I sometimes get an ad with an option to send feedback. The options are in English, although not one of them offers the feedback that's really needed: conspicuous by its absence is the WRONG FUCKING LANGUAGE YOU HALF-WITS! option.
The same language problem applies to the landing page, or whatever YouTube calls its start page. It's often deluged with videos in German, French, or…Arabic—I mean, WTF? Even several refresh attempts simply replace that useless shit with more of the same. (grumpy)
Who needs standards anyway?
The World Wide Web Consortium's entry for setting language preferences in a browser actually goes so far as to state:
Sometimes a server may determine which language to send to you in a way that doesn't rely on the Accept-Language information. For example, Google tends to use IP information to determine the language you will receive. The browser settings discussed here do not affect content negotiation based on this kind of mechanism.
I assume that this has something to do with geolocation support for targeted advertising. Nevertheless, it's a pity when useful standards are ignored by big players. It's almost as if Google's now the new M$.