I know there are plenty of esoteric programming languages that are designed to be incredibly difficult to work with, like Malbolge, Brainfuck, and PikaLang, though programs written in them can still be compiled and run by a computer.
There are lots of constructed linguistic languages, like Esperanto and Toki Pona, but for most of those they are designed to be easier to learn or have other functional benefits. Are there any linguistic languages where the goal is to be difficult to speak, write, and understand, while still being actual languages?
Tolkien is known for writing languages for his books, specifically elfish. But instead of drafting a dictionary for elfish he used his knowledge as a linguist to create rules that allowed for ‘reconstruction’/generation of words.
In contrast, Orcish/Black Speech, was specifically left fragmented, using harsh unpleasant sounds, and agglutinative grammar that changes meaning with context instead of adding prefixes/suffixes like English or Elfish.
Tolkien also never wrote songs, poems, or stories in black speech. The inscription on the One Ring afaik is the most complex writing in black speech.
So in a sense, black speech, when juxtaposed with the intuitive design of elfish, kind of fits what you’re asking for; Albeit it’s likely not a functional language.