There's also a commercial argument here. If your website or app sells stuff, you might want to care about your audience and addressing them in their own language.
There are hundreds of millions of people around the world that aren't fluent in the local language of wherever they are. Many of them probably speak English. Quite a few of those are possibly not that poor either and have disposable income.
So, if your business is selling stuff online, you could do worse than serving up stuff in English when being informed by the Accept-Language header that that is a preferred language. It's not that hard.
There are hundreds of millions of people around the world that aren't fluent in the local language of wherever they are. Many of them probably speak English. Quite a few of those are possibly not that poor either and have disposable income.
So, if your business is selling stuff online, you could do worse than serving up stuff in English when being informed by the Accept-Language header that that is a preferred language. It's not that hard.