defaultLng specifies what your preferred language is. It is used internally for a lot of different cases, and will be used in the future. Is is used as a fallback if the current language cannot be determined for some reason.
Another example is when you try to open the
index page and you have allowIndex set to false. It tries to detect the current language from the browser and if you support the user's language, it will redirect to that version of the website. Otherwise,
defaultLng is used.