I've long been fascinated by languages that humans either could never speak or would have difficulty speaking. I think I first became interested in such when I started writing a short story with demons as main characters. I cheated at the time with the two demonic languages: I used a cypher called ROT13 for Upper Demonic and random characters ( like *$*$*&*@#^$ ) for the Lower Demonic language. Of the two, Lower Demonic fascinated me more, because in my mind it was a bunch of screeches, whistles, clicks, and a bunch of other sounds I can't properly describe, woven in a pattern that - with difficulty - a human could recognize as language and even possibly learn to understand, but could never possibly hope to speak it because the mouthparts of beings who spoke Lower Demonic were completely alien, and they couldn't speak humanoid languages. Many couldn't even speak Upper Demonic. (Bs pbhefr, Hccre Qrzbavp vf gur ynathntr gung zber uhznabvq Qrzbaf fcbxr, fb V thrff gung znxrf vg n uhznabvq ynathntr.)
Then later, in my first completed (and published) novel, "I'll Tell You No Lies," the Xazian people - who could also speak verbally - spoke with each other primarily in "chemsigs" - a complex scent-based language. But I didn't even attempt to represent that in anything other than italicized translations.
I tried my first serious attempt at a non-humanoid language with a language called shaokennah, after the species that speaks it. There are clicks, whistles, growls, and lots of other animalistic sounds in it, as well as other interesting sounds, and has similarities to Lower Demonic, but sounds more... I dunno... it sounds softer in my mind than Lower Demonic. Lower Demonic, in my mind, is a monstrous language and sounds like it's spoken by monsters. Shaokennah, on the other hand, sounds softer; feral, but spoken by species that is generally peaceful despite their similarities to velociraptors, and their language reflects that. They also have two voiceboxes, adding to the complexity of their language; and furthermore, their lips don't move much, so the majority of their words are formed in their voiceboxes. But the complexity and my difficulty explaining the sounds involved stalled my efforts to complete this language, or even get very far in it. Maybe if I had access to the kinds of sounds and sound-mixing equipment the makers of Jurassic Park had, I could in time figure out their language. But it's definitely not one that lends itself easily to being expressed in standard alphabets or words.
Anyone else experiment with languages of this nature?
EDITED TO ADD: I forgot to mention in my post before, the Carbon And Silicon storyverse has lots of AI species, many of which have languages humans could never hope to even understand without some kind of implant. And many species speak in radio signals, so humans couldn't hear it with their ears. In fact, most of the species who speak in radio waves use it as a kind of temporary computer network, "speaking" in machine code. And then there are species who exist as collectives...
Oh, and in my Mindeodean storyverse, the Zokek are humanoid but they speak in a form of machine code. They're still individuals, so it's not like the Borg, but they do have implants with which to facilitate speaking in machine code, and they can plug directly into their computers with a neural interface.
*Thinks* Ah yes... and the Na'Voom Da species in my Traipah storyverse can't speak at all without this one species that is symbiotic with them. This symbiont also acts as their "hands." Well... tentacles, really. Without this symbiont, all they have is flippers, and the sounds they can make without the symbiont are pretty much dolphin or whale song, but simpler in complexity. In short, Na'Voom Da are just oceanic mammals that look like pleisosaurs, without their symbionts.
There are suspicions that the Na'Voom Da were uplifted into sentiency by an older species, but even they can't remember.