Returns a DOMString representing the preferred language of the user, usually the language of the browser UI.
Returns a DOMString representing the preferred language of the user, usually the language of the browser UI. The null value is returned when this is unknown.
MDN
https://html.spec.whatwg.org/multipage/webappapis.html#navigatorlanguage
https://w3c.github.io/html/webappapis.html#language-preferences