What your browser calls a language, is in fact a locale. You specify en_US, not English.

That would be equivalent to no setting a preferred locale or adding all the available locales.

   <html lang=...

Okay, I already have preferred languages selected: English and Turkish. If I visit a `<html lang="tr">` page I should see date inputs rendered in Turkish culture right? That's not the case unfortunately.

See it for yourself:

lang=en-US -> https://kod.ssg.dev/localetest/

lang=tr-TR -> https://kod.ssg.dev/localetest/index-tr.html