So many get this backwards.
The languages (there are multiple, including historical languages that explain the transition into the modern languages) came first - by about 40 years.
He did not invent languages for his world. He invented a world to explain how his languages would come to exist.
I don’t know if Tolkien’s notes support this, but I always assumed that Treebeard’s Entish name was something completely unpronounceable for anyone who isn’t an ent, and “Treebeard” was a nickname that he picked for himself. Maybe because he finds it funny that other species think he looks like a tree. (I’m sure that ents look clearly different from trees to other ents.)
Edit: he says so himself.
Hrum, now, well, I am an Ent, or that’s what they call me. Yes, Ent is the word. The Ent, I am, you might say, in your manner of speaking. Fangorn is my name according to some, Treebeard others make it. Treebeard will do.
Didn’t he also say that his actual Entish name would take too long to pronounce for regular mortals to bother with?
Fangorn is my name according to some
“Fangorn” means “Treebeard.”
In Sindarin (the most common Elvish language), not Entish.
Guy who walks around the forest: Strider.
Guy who betrays everyone to side with Sauron: Sauron-man.
In my headcanon, that’s not his real name. The books were written after the facts, so I imagine the writers wanted him to be remembered only as a Sauron henchman, erasing him from history.
Really, your headcanon has some precedent in the books. If Wormtongue had written the history, he literally would’ve called Gandalf “bad news.” And in fact, Saruman’s actual name was Curumo. …uh, or Curunir. Or Sharkey, or Tarindor, or…
I mean, part of the problem is that every person (and place, and country, and river…) has like a half dozen names depending on who’s talking and what time or place they’re in. Gandalf himself is Greyhame, Gandalf, Stormcrow, and Lathspell in Rohan alone; and Mithrandir, Olorin, Incanus, and Tharkun to other people in Middle Earth.
Aragorn and Strider and Elessar and Estel and Wingfoot and Longshanks are the same person in different contexts. Galadriel is also Alatariel and Artanis and Nerwen. Legolas is Laicolasse and Greenleaf (all three of which, in fairness, mean the same thing in different languages).
And that’s before we even talk about what their names “really” were in the “original” Red Book of Westmarch, before Tolkien “translated” them to English. The “actual” sound that came out of Bilbo’s mouth when he introduced himself was Bilba Labingi, but Tolkien decided that the name Labingi “actually” would’ve sounded like the word for bag or sack to the “original hearers.” Likewise Frodo’s name is “translated” from Maura Labingi and Sam “actually” introduced himself as Banazir Galpsi.
