Ask HN: Why can't you set the language individually for most apps?

A while ago I set my language in iOS' settings to French to practice my French a bit more. Since then I noticed two things:

- Despite most apps certainly being available globally with multiple languages, they usually do not offer any option within the app to set the language to something different than the system language.

- Even weirder, some apps/services will assume that the system language is your main language and automatically set that language in your account as your main language. For services that can be accessed both on mobile/desktop, that sometimes leads to emails from that service switching back and forth between languages.

Why is that? Especially for the first point: since many apps are in principle available in multiple language, why not give the user the option to choose the language regardless of the OS settings?