> It’s not the case for a very large portion of people around the globe.
Yep, even people who speak one language might put their phone or browser into another language for learning (immersion) purposes but need your important (e.g. banking) application to be in their native language regardless.
Yepp, just look at other responses here in this thread. People often assume that there is a system language that is used in the browser; that is only partly true, browser settings allow to specify multiple languages with given priority. It is just usually the default system language, until you configure it. The browser might not even use the system language in that setting at all if you configure otherwise.
> We used the device's language as the app's language, because that makes sense, right?
One of the mistake devs do all the time is assuming users use _one_ language. It’s not the case for a very large portion of people around the globe.