Which author created Middle-earth?

Tolkien, and he built the languages first. He was a philologist by profession - professor of Anglo-Saxon at Oxford - and invented Elvish languages for pleasure from his teens, then concluded that a language needs speakers, a history and a mythology to feel real. The stories exist to give the languages somewhere to have come from, which is the reverse of how almost anyone else works.

Middle-earth was also meant as a replacement. He thought England had lost its own mythology, unlike Finland with the Kalevala or Iceland with the Eddas, and set out to construct one - which is why the tone is elegiac and why so much of it concerns things being lost, diminished and remembered rather than won.

The Lord of the Rings took twelve years and was published as three volumes only because of paper costs; Tolkien objected to the term trilogy his whole life, since it is one novel. He was also firm that it was not an allegory of the Second World War, noting that if it had been, the Ring would have been used against Sauron rather than destroyed. What he did allow was applicability - that readers bring their own wars to it - which is a distinction he cared about a great deal more than most of his readers have.