> The Voynich Manuscript: clearly a language used to share thoughts, even though only the author (presumably) understood it.
The Voynich Manuscript has been referred to as a "personal shorthand" rather than a "language" quite often. "Language" is a specific term, and "being used to exchange ideas between people" seems to be part of the definition as people care to use it.
We do call large systems of grammar+vocabulary invented by one person "con-langs", which suggests they are "languages" — but we only call them that when those languages have usage guides published, such that other people could theoretically attempt to speak them. Before that happens, we don't really have a term for them. They're private thoughts in the creator's head. Do you need a term for that?
(What do you call hypothetical semantics underlying the random runic scripts used to decorate buildings in sci-fi movies, that only the authors know the meaning of — if there ever was a meaning at all? Intuitively, "language" is not quite the right term there. Even if there is a fictional language in that fictional setting, the representative samples of writing aren't necessarily valid samples of it — especially if the authors never bothered to actually construct a language for those samples to be sampled from.)
> the "artist" shuffling around a pile of salt whilst on his knees for hours each day
My argument there is that a lot of people are conflating something being "bad art" with that thing being "not art." But they don't do it all the time. They still use "art" to mean "art", as well as using it to mean "good art." They'll use "artful" or "evocative" to talk about "good art", but they'll use "artistic" to talk about any "art", good or bad.
Art, like writing or music, is an act of communication. Art is seemingly described as "good" when it's powerful; that is, when it successfully communicates something, perhaps impactfully.
That has nothing to do with aesthetics, of course. Awful images can be good art, if they're powerfully awful images.
Bad art, then, is powerless: it fails to communicate. But this still doesn't mean it's not art.
Art isn't opacity; doing a really bad job of making art doesn't make the thing less art. It makes it lesser art — lesser according to some societal ranking function — but that's not the same thing as becoming less like what people mean when they say "art."
People who are speaking unintelligibly (perhaps because they're very drunk, say) are still speaking, rather than "making noises." They're still attempting to communicate an idea through the medium of sound to others, and that's what makes that noise they're producing "speech." Intelligibility doesn't come into the question of "is it speech." It only comes into the question of "is it a successful act of speech."
A human born in the wild, though, with no exposure to other sapient minds capable of engaging in exchange of ideas, isn't "speaking" when they make mouth noises. They don't even have the concept of "speech." They're not attempting to communicate any ideas to others. There are no others.
Art is, AFAICT, the same.