What is the difference between a default language and other languages?

The address that is selected as the default one, will be displayed fully detailed to users at the company profile page. If the default address contains a location, the location will be displayed at the profile page too.  

Also, if you select an address as default, a “Contact us” form will be placed at the bottom of the company profile page. Users will be able to contact (Send email) you directly by the inserted email address in the default address.