Gordon R Dickson 
Magnificent Wilf [EPUB ebook] 

When Earth is contacted by galactic civilization, our heroic couplet – diplomat Tom Parent, and his linguist wife, Lucy – prove themselves to be just the pair to tour the galaxy representing Earth and learning the whys and wherefores of galactic civilization. There’s only one tiny catch to this grand tour – on Tom and Lucy’s performance hinges our acceptance by the rest of the galaxy as a civilized world rather than as ward of some more "advanced" species. (You don’t want Earth to become a Galactic ward.).