Tolkien devoted countless hours to the growth, cultivation, and evolution of all of his languages, but there are a few that do not change. The language of the Valar, Valarin, is an almost unchanging thing that came into the world with them. “The speech of the Valar changes little; for the Valar do not die” says Tolkien in Lhammas, and before the making of the Sun and Moon, Valarin did not change (B.1 184). With this language, the Valar described all that they created. Elves learn this unaltered speech when they awaken, but “since they changed it from the first in the learning” they develop new words and languages and continue to do so (Lhammas B.1 184). This is not attributed to their mortality though; it is because elves were made with a certain inventive quality. They delight in the “making of words” (Lhammas B.1 184). From here, all languages of Middle-Earth, save that of the dwarves and orcs, are at least partially derived.
The names assigned to the things of the world by the Valar in Valarin are curious. The Valar, having created these things, know the traits of the world better than other person or thing. As names are adjectival, it seems that there exist True Names for everything, describing every trait, every facet of these items. These True Names do not give power in that they allow for control over an object, but they give power in the form of wisdom and knowledge. Knowing the Valarin name for something allows for a complete understanding of the thing. This is not true for men, elves, or the Valar themselves since they are of Iluvatar. They cannot know their own True Names or those of anyone else, as it is impossible to know oneself well enough to be fully descriptive, and it is even more impossible to know another fully enough to even attempt to Name them. Eru is the only with such power and the language to do so. The same may be said of orcs. As perverted and/or conscripted humans and elves, the orcs cannot be named by Morgoth though their speech is his perversion of Valarin. The True Names of the dwarves, however, are completely different. The dwarves were made by Aule in anticipation of the elves and men. These dwarves are the creation of the Vala Aule and can be named by him. They were also given language by him, though he did not teach them Valarin. Aule taught the dwarves a language of his own invention, and it is a descendent of this language that they speak between themselves for all recorded history. Dwarves are said to have True Names and guard them as personal secrets, never revealing them to any outside of the dwarf race, but this does not quite hold up. In order for this to be true, the dwarves would need to know Valarin and be personally enlightened by Aule as to their Name. Dwarves may have private names, but they can in no way be their True Names. This matter is further complicated by the adoption of the dwarves by Iluvatar. Before their adoption, Aule must have known their True Names. After, it is unknown whether their entire being is known to Aule, but this is dubious.
It is interesting to consider the implications of Khuzdul being a constructed language. At some level, all language must be constructed, but where there are gods in play, it is interesting to see one construct a language for their creations. The construction of Khuzdul reinforces the idea that the dwarves are a constructed people. They are as natural as the plants and birds and rocks and things, but they are not quite on the same level as humans, elves, or the Valar. They are a subcreation, and as such, are not capable of subcreation themselves. They were not given a spirit by Iluvatar and cannot participate in the subcreation of the world; their craftsmanship has not the quality of art, though it may be beautiful (their lack of spirit is according to the Lhammas, Tolkien later revised his work, adding the adoption of the dwarves by Iluvatar and their imbuement with spirits). The relationship between the dwarves and the earth is not commented on much since canonically most accounts are translations by Tolkien from uncovered manuscripts like the Red Book of Westmarch. It is known that they are a people fond of rocks and metals because they slept under the mountains until the awakening of the elves and that they are not fond standing on anything other than solid dirt or rock. Not much is known of their religion save their relationship to Aule. Little else is known.
Having devolved far enough, it is time to return to the unchanging languages. From Valarin and the craftiness of the elves comes Quenya. After its development and the sojourn of the elves to Valinor, the original form of Quenya does not change. While this is not explicitly stated by Tolkien, both the Tree of Tongues from the Lhammas and by his omission of any changes point toward the absence of change of any significance. Words may have been added to account for new ideas or things, but there was no true change, even in dialect. The same is true for Linadrin, Eldarin, and Telerin. After moving to Valinor, these languages became stagnant, and in the eyes of Tolkien (at least in the manner in which he views human languages), they were dead. All these languages underwent significant changes while still in the East, especially after the creation of the Sun and Moon, but life in the Undying Realms stagnates everything. The implications of this – the stagnation of elvish culture and language – are interesting. It shows that the time of the elves in Valinor is over (and as all elves eventually die or go to Valinor, this is eventually true of all elvish languages and cultures). The First Born become as unchanging as the Valar themselves and thus have an elevated stature from their original. From there they may sing and dance as they have for thousands of years, hopefully returning in England’s time of greatest need.