Wall Around a Star

He was a reluctant passenger on a voyage to save the galaxy...
Butterflylike aliens had brought Earth into the galactic culture. But she was a poor relation, valued only for the living human human bodies she rented out for whatever purposes her nonhuman customers desired.
Then Cuckoo was discovered. Millions of miles in diameter, less dense than air, it had a solid surface that was home to many races - including a species of Man. And that was odd, for Cuckoo was from another galaxy!
Suddenly, one human, a linguist, became very important. If Jen Babylon could solve the mystery of Cuckoo's records he might raise humanity's standing among the older races - but he might also save the galaxy!