Add as friend means you are able to message him-her always , or able to chat with him-her through the chat ...and if you really serious in learning a particular language , I would prefer add as a friend , because the contacting and keeping in touch will be more easy of course !! you could exchange E-mails , Skype , Fb or what ever you want ..
While following someone, is just a way to keep watching the updates of someone ..
that's it =)
Good luck .