Zuko8va, that is not from the actual show, though. I doubt we are meant to think that the letter, if it even exists canonically in-universe, is written in actual English. Looks like it's from the Nickelodeon website or something like that. In this case, I'd say this would be a translation too for our convenience's sake.
I've wondered about these things too. I'd love to know what the elements actually are, for instance. Because they seem to be a part of the nature of the very universe, a part of their fictional "physics" in a way. I know the lion turtles had them first and gave them to the humans, but what about beyond that? The lion turtles are also called "the Ancient One(s)" and seem to predate even humans AND spirits.
@FoxDaryl even though crediting you would be polite, there is no law that states ideas themselves are copyrightable. They're not.