This Extra Field Type is available in Profile Builder Free, Basic or Pro.
You can use this field if you want to save the the current language that is displayed to the users when they register. Then, for example, you can use the Compare shortcode against this field in order to write the emails in different languages.
This field is hidden for the user in the front-end. It will automatically detect the language selected in the Language Switcher. Only the Admin can see the field in the front-end for each user.
This is how the Edit Profile Form of a user looks like viewed by Admin: