Yes you do. Plenty of people using an OS in whatever language can barely speak a word of that language. Most commonly they'll use an English OS for whatever re

Yes you do. Plenty of people using an OS in whatever language can barely speak a word of that language. Most commonly they'll use an English OS for whatever reason and want to browse websites in their local language. This is why you'll find that a lot of website language detection tends to discount English specifically. I.e. if your Accept-Language is "en" in France they'll probably give you French, but if it's e.g. Japanese in France they might think that's a "real" setting and give you Japanese. Or, as I'm sure a lot of non-English speaking technical users here can attest to, they like to use an English OS UI on their laptop or phone, but speak the local language and want browsing a local website to just work. Non-technical users do that too, but have no idea how to tweak their Accept-Language independently from their OS language. Really, do armchair commentators here think that programmers at major websites like Google that have enough data to know how people are likely to switch languages from the default haven't thought of just implementing what's literally the most simple thing you can do, which is to obey Accept-Language?