To the point raised earlier, to the effect that Klingon is a system and idea and therefore all circumstances of its origin or context are moot, it cannot be copyrighted, based upon:
The Copyright Act does not extend protection in a work to “any
idea, procedure, process, system, method of operation, concept,
principle, or discovery, regardless of the form in which it is described,
explained, illustrated, or embodied in such a work.”
A thought experiment. For a moment, require that Klingon be called [your least favorite politician]speak. Require that the backstory "cultural" concepts from which the "uniquely expressible only in Klingon" qualities arise which come from Trek IP, be replaced with only concepts from the backstories and sayings of said politician.
You get to keep the dictionary and script and grammar, except of course the Trek-IP originated idioms need to be shifted to the politician's tropes, and at your gatherings you need to dress up like the politician

.
Is the language still Klingon?
If not, why not?
And if not, how can it claim to be anything other than an extension of Star Trek story elements?
I do not study Klingon. I couldn't answer the above question. But I would like to see anyone speaking Klingon answer it (without dismissing it as irrelevant).
The AC filing notes that although important literary works have been translated to Klingon, the meaning has to change to conform with backstory concepts about Klingon culture. Japanese MacBeth was first translated with all the cultural references shifted. But Japanese had its own independent cultural history to draw upon. Klingon it seems only has stories and character qualities written by the studios.
In fact, the AC filing repeatedly references Klingon's unique qualities, and every one of them are concepts created by Trek. Fans are adding "Klingon culture" backstory to this language (the AC brief for example refers to the Klingon Language Institute quarterly academic journal studying Klingon linguistics, language, and
culture). Changes to Klingon must be approved by a very small group of keepers of the Klingon flame. So is all this extension kept in alignment with the Klingon backstories from Trek, and carefully amplifying it, or has Klingon disowned those backstory elements and become something else, a language that can stand on its own without Trek IP?
You may ask why does this matter. My view is if fans are extending the studio - created language with stories and Klingon "culture" concepts taken directly from the foundation of "Klingon culture" stories created by the studio without participating in studio copyright, and saying its ok because "language is a system [loophole]", then it seems to me little different from Axanar claiming that copyright does not apply to them, even though their work is 100% dependent on freely and deeply using the backstory elements of the Trek universe, because there is the "loophole" that fan films are not in practise required to participate in licensing copyrights on Trek, and because Axanar is the true keeper of the real meaning of Trek.
With regard to copyright, I therefore believe it is a misapplication of the definitions of "system and idea" to say that Klingon is only or mainly a system or idea promoting a useful science or art (U.S. Const. Art. I, § 8, cl. 8., as cited by the AC brief). Klingon, to me, is a story element of Trek both now and in its origin, because as far as I can tell from what people say about it, it originates from and seems to be kept in logical continuity with the "culture" of the stories written by Trek authors, and cannot be separated from those story elements without losing its identity.
Of course, this is all just my take, and I am not an attorney. As I said earlier, CBS would be wise to limit their Klingon claims and not enter this sideshow. But if they do, I think this is a reasonable interpretation of why their argument might have merit in the case of Klingon.