When you are writing, you are free to create anything.
Worlds, people, wars, relationships, even languages.
What You Create Is Yours
Not only is this poem something that he created, but it is created using something he created. He could not have written that poem if he had not first build a new and beautiful language to write it in.
You Can Create Anything
When we write, we tend to put contraints on ourselves, even if they don’t exist.
Tolkien and others (such as Marc Okrand, the American linguist who created Klingon) have shown that you don’t even need to be constrained to existing languages.
Whether you invent your own language or not, you can create sentences that sound as beautiful as Tolkien’s Elvish poem. Don’t be constrained.