I'm confused, if the operating system is set to use some date format why not use that? You don't need to even reveal the format to Javascript since the input sh

I'm confused, if the operating system is set to use some date format why not use that? You don't need to even reveal the format to Javascript since the input should work without JS. Language preferences are a totally separate thing, although they could be used as a fallback if date format is somehow not offered by the operating system. E.g. de-CH for a swiss date format, de for a german date format.