I am not a lawyer, but I have done some research into copyright case law.
If any languages could be said to be intellectual property then surely programming languages must be numbered among them. I have studied a few of these, and I have never once had to pay anyone for works I wrote in them. My efforts are trivial at best when we consider that the most advances programs are collaborative efforts that rival encyclopedias in length. Perhaps someone does charge for their use, but I have not seen it in my experience. If so many programming languages which are certainly closer to traditional intellectual property than a constructed language for use in human communication has ever dreamed of being are open to the public how can Paramount claim to own Klingon? If ever anyone could have claimed constructed languages as property it was the creators of the first programming languages, and since they have thrown their inventions into the public domain how can any constructed language now try to put that genie back in the bottle?
Sun, Microsoft and other companies have sued to protect the intelletual property(abbrevated IP) rights of computer languages. As IP owners of the language they chose what rights to allow and which to share. many of the original programming languages were developed either by Bell Labs, University student groups or hobbyists. Bell Labs which owned the rights to the C language written by Dennis Ritchie had decided not to defend many of its copyrights as it was already on the watchlist as a potential monopoly and ultimately was forcibly broken up into "Baby Bells". The Original Basic was developed at Dartmoth College, Lisp at MIT, COBOL was created by a committee of government and business techs and funded by the Department of Defense, therefore was a public standard. ALGOL was also developed by a committee of European and American interests. For the most part there just wasn't an option to copyright the language, since the languages were written to make it possible to write applications for hardware. The applications and the hardware was where money was made so most of the languages were made freely available and free of any licensing restrictions. More recent computer languages particularly those by Microsoft and Sun, tend to have more restrictive licenses. Although it's important to note that for a computer language to be successful they need to attact people to use it, so high barriers to entry such as license fees are difficult to implement... Still there are some APIs which can be viewed as extensions of Programming languages wbecause they extend and make them more powerful, some APIs have very restrictive licenses and include licensing fees that may even be based on a percentage of profits.
But all of that is irrellevant because each author is free to exercise or waive their copyrights as they see fit. If the first 100 programming languages were all free, there is nothing legally stopping the owner of the 101st language from charging people for using his work to create a commercial product. Though it must be a pretty good language to beat out the 100 free ones.
On the other paw the works that describe Klingon are copywritten and so using those works to create other works, are legally considered derivative works. Works that would not be possible without the previous works. Klingon is not a natural language. Okrand did not document existing facts as if he had comiled the names and telephone numbers of real people and published a telephone directory. Okrand invented a language form scratch. Contrary to claims inside the book which claim Okrand merely documented the Klingon language from interviews with a particular Klingon speaker, the Klingon Dictionary and other subsequent works are a creative work and are therefore subject to copyright.
Secondly, is not each written work in a language that could be copyrighted itself a piece of intellectual property? Is it a piece of intellectual property due to the words it uses, or is it s piece of intellectual property due to the story it contains? I must say that the evidence to me seems to fall towards the story and not the written work itself being the piece of property. If I write a book in English which is subsequently translated into German, I still own the rights to the book not the person who translated it. Paramount would be asserting its claim to an independent author's intellectual propert simply because of the language in which it is written. This would be akin to the translator owning the book instead of the author.
You are partially correct, you would hold copyright over your original expression in
tlhIngan Hol, but as a derivative work, you still would need to acknowledge the previous work and get permission to use Okrand's and the KLI's materials to create your work if you wish to publish and sell it. KLI applied for and got blanket permission to use the Klingon language in their works. Anyone who wishes to earn money from a work that uses the Klingon language in any significant capacity would also have to gain permission. Whether that costs money and how much is entirely up to the IP owner(s).
If you can't get the copyright thrown out as invalid (as you can't with the above argument), the only ways around a vaild actively defended copyright are to wait for the copyright to expire, or to prepare a clam of fair use. Fair use is not a right, but a legal defense. That means even if you have a perfectly solid fair use claim you can still be dragged into court in order to present your defense, which could quite quickly cost you more in legal fees than you would have made in sales unless it's rather an amazing work. If your work is entirely in Klingon as opposed to if you only used a few words in an otherwise unrelated book, that is one of the four big things judges tend to look at that will not go in your favour. If you intend to make money from your derivative work, that is a second of four points against you. Ultimately it's up to a judge to decide, but with two big strikes against you... I wouldn't take my chances. You really should consult an attourney before attempting such a work and I suspect they will help you negociate to get permission to use
tlhIngan Hol as documented in the copyrighted works.