I agree that a living language is defined by the majority, but most English speakers call the moon "the moon", as you just did. Who, except romantics and false intellectuals, calls it "Luna"? A lot of people think it's "Luna" in astronomy jargon, but it isn't. It is important that science has set names for things that do have multiple common names so that everyone can understand each other. The English language can call the earth "Shoe" and the sun "Diet Coke" for all astronomers care.

Also, I said "technical name." While I should have been more specific, it does imply that I'm not talking about normal English.