This property describes the system specified language and country code. It uses the same language values as the navigator.language property.
Refer to the Language codes topic for a list of the available codes.
The most likely default value you will encounter will be "en" which signifies English but does not make distinction between UK and US variants. The value is supposed to be case insensitive.
Note that some platforms will yield a language and country code while others may only yield a language code.