I'm newbie. I know “中国“ means "the country on the Earth's center", so, "the Middle-earth" = ”中国“？ I mean John Tolkien's universe.
Translation based on Vala rather than middle-earth.
The Valar [ˈvalar] (singular Vala) are characters in J. R. R. Tolkien's legendarium. They are "gods" or "angelic beings"1 subordinate to the one God (Eru Ilúvatar); they are the most powerful of the Ainur who chose to go into the World (Arda) and complete its material development after its form was determined by the Music of the Ainur (Ainulindalë). They are mentioned in The Lord of the Rings, but were developed earlier in material published posthumously in The Silmarillion and The History of Middle-earth.
where the Chinese article translates to:
维拉 Valar，是英国作家约翰·罗纳德·鲁埃尔·托尔金的史诗式奇幻小说《精灵宝钻》中的种族，意思是“阿尔达大有力量者” Powers of Arda，女性的维拉则被称为维丽 Valier。维拉是精灵和人类眼中的诸神，他们是伊露维塔的意念创造出来的埃努，其中十五位力量最强大的埃努，协助伊露维塔统治阿尔达，被人类和精灵称为维拉。后来其中一名维拉被除名，成为了天魔王。
Middle-earth is the fictional setting of much of British writer J. R. R. Tolkien's legendarium. The term is equivalent to the term Midgard of Norse mythology, describing the human-inhabited world,1 i.e. the central continent of the Earth in Tolkien's imagined mythological past. Tolkien's most widely read works, The Hobbit and The Lord of the Rings, take place entirely in Middle-earth, and Middle-earth has also become a short-hand to refer to the legendarium or its fictional universe.
and Chinese equivalent
alternatives to 中土大陸 include: 中土世界 and 中洲.
The History of Middle-earth is translated as 中土世界的历史
The Peoples of Middle-earth is translated as 中土世界的民族
see wikipedia's: 中土世界的歷史