Was Tolkien really the first writer ever to develop conlangs or that was already a thing in sci-fi and fantasy before him? Were there many babel fish/tardis telepathic translation explanations for the "all aliens speak english" narrative convenience in early/pulp sci-fi or this was not acknowledged at all?
No, Tolkien was absolutely not the first, though he was the first to do so with the absolute detail he would be known for.
Edgar Rice Burroughs created an entire dictionary for Mangani, the language of the semi-human anthropoids that raised Tarzan in the Tarzan novels. While inventing fake words for fictional cultures has no origin point, Burroughs's Mangani is easily the first to reach this level of detail, a near complete, though simple language. Spoken by apes, there was no need for a huge vocabulary: they didn't even have words for human being. Tarzan, in Mangani, incidentally, means "White Skin," and white men are referred to by Tarzan as Tarmangani (light folk), with black men called "Gomangani" (dark folk).
(Incidentally, it was explained the reason Tarzan never got with any black native girls growing up despite his lack of any Western-style race prejudice is because in the region of the jungle he grew up, the Bantu tribes had a habit of chewing betel nuts for energy, a malodorous habit that was offensive to Tarzan's hyperdeveloped sense of smell. Though he engaged in the habit himself for a brief time in Paris when he despaired at losing Jane, the only time in Tarzan's life he ever drank absinthe, Tarzan thought much the same thing of Westerners who smoked cigarettes, which he found retch-inducing.)
Burroughs's chief influence in creating Tarzan was Kipling, who's Jungle Book used actual Indian languages Kipling was familiar with to convey a sense of a different location. It has often been said about Kipling's influence on Burroughs, that while Kipling was a better writer, Burroughs was the better romantic and daydreamer. Burroughs had no knowledge of the languages from the region he wrote about and had never even visited Africa when he wrote Tarzan of the Apes in 1912, but as usual for Burroughs, he made up for his lack of erudition and education in comparison to Kipling with his sheer imaginative power.
Although it is worth noting that modern languages in India have changed from Kipling's time. For example, in India today, bagheera in most places would be more likely to refer to a tiger instead of a black panther. Sher Khan is of course derived from Khan, a title of kingship, and Sher/Shir actually means lion, not tiger: it comes from Persian, interestingly. A city in Iran is known as Shiraz (place of the lion) and in Chinese, lions (not native to East Asia) are referred to by the Persian loan word shi 獅, carried across the Silk Road. The very famous Chinese habit of "Guardian Lions" placed before doorways actually was brought by Persian traders, and the earliest examples in China are of Persian manufacture, a sign of the incredible association of the world's two greatest civilizations.
(Interesting fact: watermelons in China, brought from Persia, are actually known as 西瓜, or "Western Melons." Even though these days, they are grown in China and don't need to be imported.)
Because it was used in movies and Mad magazine, one term from Burroughs's invented Ape Language, Kreeg-Ah, "broke containment" and is actually pretty well known even to non-Burroughs readers. Although it is commonly believed to mean "I kill!" or something like that, it actually means something like "danger" or "watch out!" When Tarzan used it to say "Look out, I will kill" it becomes kreegah bundulo.
The Mangani who raised Tarzan were not chimpanzees or gorillas or any kind of known ape, and they explicitly referred to gorillas by another term entirely (Bolgani). They were something physically closer to a Sasquatch but with semi-human intelligence and something close to a language and even rituals like the dum-dum dance, created at a time when gorillas and chimpanzees had just been discovered and anthropology knew very little about them. A lot of fans of Tarzan, including superfan Philip Jose Farmer - have speculated the Mangani were actually something very much like Louis Leakey's ape prehuman ancestor, Paranthropus. In short, they were less a species of ape and more a kind of cryptid, an idea the later books in the Tarzan series supported by saying the Folk, the Mangani, were near extinction and encountering them was extremely rare.
Many artists have drawn the Mangani apes as looking completely different from any known species of ape. The great Russ Manning was the first to draw them as having huge white muttonchop hair on their faces like a colubus monkey, something that both Neal Adams and John Buscema adopted in their Tarzan comics due to the tremendous stature and influence of Russ Manning in the comics world. Despite that, the "white muttonchop sideburns" are nowhere near described in Burroughs, just like Tolkien never explicitly said that Elves had pointed ears. (He never did. Read it again.)
Frazetta, who said the scene of the frenzied ape Dum-Dum ritual was his favorite scene in the book, used to draw them as a mix between a gorilla and a huge baboon:
If you're at all interested in the Mangani language (taking a trip to Opar, where the language is still spoken, perhaps?) there was a great illustrated dictionary by Jairo Uparella.
Was Tolkien aware of Burroughs's constructed language? Yes, but it was not an influence. Unlike Lord Dunsany, E.R. Eddison (who Tolkien actually knew personally) and H. Rider Haggard, who Tolkien acknowledged as influences and stated his admiration for them, Prof. Tolkien adamantly denied that Edgar Rice Burroughs had any influence on him, and I agree with that. Tolkien - rather testily - denied that Burroughs's giant spiders from the Barsoom and Tarzan series inspired Shelob, and in personal correspondence said that "I have acquired a loathing for Burroughs's Tarzan that far exceeds my loathing for spiders."
Rather, where Tolkien's achievement truly was unique was that Tolkien was the first to create artificial languages of the complexity of a real language. Tolkien was a genius with a planet-sized brain who spoke Ancient Welsh and who not only spoke the Ancient Germanic language of Goth, but actually wrote poetry in that language.
You know, it really, really rankles me that people say that I am a "hater of Tolkien." Absolutely not, never have and never will. I was obsessed with Tolkien in college and wrote tons of messages in Dwarf runes. I read Lord of the Rings to the point of memorization.
Rather, I think the reason that this label has been hung on me is because I deeply resist the idea that Tolkien was the central, foundational figure of fantasy fiction. Do people actually think he invented the fantasy genre? Absolutely not! The genre began a couple generations before LotR was even published. He is a towering figure in it, but he did not "start the fire," and he wasn't even the first academic to do it, either, to bring that level of academic rigor to it...that would be Andrew Lang. Incidentally, Tolkien's best friend and Lord of the Rings beta reader Roger Lancelyn Green (who famously, while being read LotR prior to publication, got up from his chair and said "oh God, Ron...not another fucking elf") wrote a biography of Andrew Lang.
For example, D&D creator Gary Gygax said that Tolkien had a surprisingly minimal influence on D&D, and I actually agree with that completely, since Gygax was a product of the pre-Tolkien tradition of fantasy fiction like The Moon Pool and Hour of the Dragon. I mean, if the Wizard class actually was inspired by Tolkien, wouldn't it look different? Rather, Gygax notoriously disliked Tolkien and only included him in the D&D appendix of inspirational fiction as out of a sense of completeness and obligation; Gygax was more inspired by Robert E. Howard and Abraham Merritt. D&D Elves for example, are far more like Poul Anderson's concept in "The Broken Sword," as they have no true souls as humans do and reincarnate into nature after death, and do not need to sleep. Incidentally, here is the first ever illustration of Elves in D&D, who are bearded, large nosed childish goblin-like beings like in children's folklore, and nothing like the immortal and stately beings in Tolkien:
