I loved some of the ideas in this thread, but I had certainly been thinking about planar languages for some time beforehand. Now I feel a need to get some of this stuff written down. My ultimate goal is to go through all the major D&D languages (or at least the ones relevant to Planescape) and provide some of the following:
Linguistic overview of the language (what's cool about the language?)
Phonology (what sorts of sounds dominate the language?)
Original text (using Unicode alphabet, write something to show what words look like, and provide a translation)
Script (the really tricky one: make an image of the script of the language, using non-typable characters, and maybe the same text as category three)
I'll write some of the linguistic overviews that I've started. None of them are really what I'd consider "complete," though they should be because they're getting too long.
Modron
The Modron language is a peculiar one, fitting for the unfathomable beings who speak it. There is precisely one dialect, spoken by all modrons of Regulus, from Primus on down. Modron is an alien speech for mortals, notoriously difficult to learn, but its rudiments lie in unwavering regularity. It may seem like a maze of declensions, case, and affixes, and to hear it spoken by the modrons themselves, it appears to be nothing more than clicks, whirrs, and whistles at varying pitches. The written form of Modron connects conceptual nexuses through various strings of truth-falsity validations so as to provide mathematical expression to the words.
The most fascinating aspect of Modron is its vocabulary. A language so complex might be reasonably expected to have a correspondingly vast set of words and constructions. In actuality, though, Modron features an exceedingly minute vocabulary, and the language is constantly being revised by the Administratum of Language. This team of specialized octons and one nonaton work to steadily purge the language of redundancies, viewed by modrons as among the worst crimes of which a language can be guilty, and one of the most deplorable features of non-modron languages.
As a point of illustration, no antonyms exist in Modron. Instead, an antonymous affix is attached to the word, forming two words with one agglutinative base. The word for "chaos," for example, is rendered as "un-law." Additional affixes exist to mark degrees of intensity and every other variation imaginable, if "imagination" were a trait that could be applied to the modrons.
"Only the modrons would think that they could un-define chaos."
-Ciran Pross of the Doomguard
Some have argued that this basis for a language actually makes for less efficient speech, that referring to an antonym by it's opposite's name is unnecessarily cumbersome and that surely that embodiment of efficiency, the modron hierarchy, would have designed something more streamlined. At the risk of digressing further into philosophical topics, I hope to address this peculiarity of Modron. The answer lies in the modrons' devotion to no only Law, but in particular Order. Modron society does not permit for there to be deviation of thought or deed, and therefore the language is designed with the intent of limiting every concept to one possible expression. Modrons, without the words to express a chaotic concept, are unable to conceive of chaos, as they are unable to conceive of rebellion, independence, or subversion.
Three-Parabola of the Administratum of Language writes in its Linguistic Discourse 11L that "The Administratum carefully reviews all aspects of its syntactic jurisdiction. Although other races typically find its precepts difficult, the current execution of the language achieves a density of data efficiency unequaled in any form of communication shot of direct, psionic contact." Perhaps, however, a more illustrative translation might read:
[REVIEW; present=true; [DILIGENCE; adverb]] [ADMINISTRATUM; case=nominative; plural=false] [CONSTITUENT; quantity=ALL; plural=true; for which referent equals [AUTHORITY; degree=unPrimus; [STRUCTURE plus LANGUAGE; case=genitive]]]; Universal_state=true; that: [CONSIDER; present=true; [TYPICAL; adverb]] [COLLECTIVE; case=nominative; plural=true; plus BEING; modron=false] [PRECEPT; plural=true; case=possessive; condition equals [UNSIMPLICITY; adjective] {conditional statement {For component [DO; noun; for which condition equals [PRESENT; adjective] /for which referent equals [LANGUAGE; modron=true]], value equals [QUANTITY; plural=false; for which referent equals [EFFICIENCY], and argument equals [DO; noun; for which condition equals [PRESENT; adjective] /for which referent equals [LANGUAGE; modron=false]], the value of the component is greater than the value of the argument. Exception equals [COMMUNICATION plus PSIONIC]}}
What's more, the near-uniformity of the language across speakers allows for the semantic affixes to differentiate by exceedingly slight variances in each word without confusion. At least, without confusing the modrons. This has the effect of making Modron nearly impossible for outsiders to understand, as well as allowing modrons to communicate ideas as labyrinthine as the above quote at a rapid pace by using brief words.
The Sensate poet Winnam an-Shal once famously tried to write a book of poetry in Modron. Though his first effort succeeded in fitting his entire "Ode to the Spire" on a single parchment line of condensed symbology, it is said that only an-Shal's famous task dedication kept him from giving up on poetry altogether in his boredom. Perhaps this same diligence is what enabled him to achieve such a high level of proficiency in the language. To his credit, his efforts were remarkably advanced.