In short if your location has a different default language setting than what you are sending, what you are sending is probably what you really prefer.
...or because your browser defaults to sending en-US even though you're using a Danish-localized browser on a Danish operating system, as described by the parent comment.
It is however not what I experience when I look at my Danish localized firefox because it sends da-DK by default, and my english browser sends en-Us, because I took the effort to make it so, because I don't much like the Danish language for UI.
I admit this is just a quick look over of the different language browsers I have on my system. What do the different language browsers on your system do that you feel so confident in telling me to pay more attention to the parent comment?
Indicating that "the user's actual language" and "en-US" are different. Yes, I extrapolated this to both the browser and the operating system having a different language, maybe it was just one or the other.
> What do the different language browsers on your system
From the parent comment:
> Google has been doing this for almost 20 years. [...] The rationale (which I still don't agree with) was [...]
I don't have any 20 years old browsers to test with, I'm afraid.