Literary types may act proud over the slang in Burgess’s A Clockwork Orange or the linguistic hybrids of Joyce’s Finnegan’s Wake but fanboys and girls everywhere are made of sterner stuff. Mention Science Fiction or other fantastic writers who have created languages for their worlds and J. R. R. Tolkien immediately springs to mind. Being a professional philologist, Tolkien had the edge on most. His languages of Elfish and Dwarvish along with others made the filming of his The Lord of the Rings more convincing. A similar verisimilitude can be assigned to the Klingon language in the Star Trek universe. These are languages you can actually learn, though finding others to speak it with might involve going to an SF convention.
Rudyard Kipling can take credit for anthropomorphizing the different animals of the jungle and giving them a shared language. Edgar Rice Burroughs took this a step further and created a lengthy primer for Tarzan’s jungle world. This language is that of the Mangani, or ape folk. ERB did this starting in 1914, so even Tolkien must bow to him chronologically.
One of the brilliant (seems obvious now) innovations ERB did was to have the cavemen of Pellucidar, the cavemen of Nu’s Niocene era, the apemen of Opar and the monkey-tailed men of Pal-U-Don speak a form of related Mangani. If Tarzan had ever got to Caspak (Russ Manning took him there in the comics!) I am sure he would have found them all speaking Mangani to different degrees. This gave ERB’s worlds a nice connection that he actively cultivated.
Dell comics offered this two page primer in Tarzan’s Jungle Annual #2 (1953) so you can sharpen your linguistic skills for the next time you’re hanging around the jungle.
You state that ERB began his ape/manganic language in 1914. That is the date that TARZAN OF THE APES was originally published in book form by A. C. McClurg. But TARZAN OF THE APES was first published in 1912 in the October issue of THE ALL-STORY MAGAZINE. Burroughs added to his ape language after he wrote TARZAN THE TERRIBLE by adding words he had created for the Hodon and Wazdon humanoids.
You are right on both accounts.