Alignments are a prominent trope in the Greyhawk setting − even in the sense of forming cosmic factions. Each alignment has its own language! These really are factions.
To translate 1e into 5e, I suggest the following matrix.
Alignment (Astral Dominion, Creature Subtype): Language
Good dominions:
• Lawful Good (Mount Celestia, aasimon): Celestial
• True Good / Neutral Good (Elysium, angel): Elysian
• Chaotic Good (Libera aka Arborea, advocate): Liberan
Neutral dominions
• Lawful Neutral (Mechanus, modron): Mechan
• True Neutral / Neutral Neutral (Outlands, primordial): Primordial
• Chaotic Neutral (Electa aka Limbo, slaad): Electan
Evil dominions:
• Lawful Evil (Hells, devil): Infernal
• True Evil / Neutral Evil (Waste, yugoloth): Deepspeech
• Chaotic Evil (Abyss, demon): Abyssal
The 5e astral plane is a realm of thought: concepts, ideas, knowledge, dreams, ideals, cultures, and beliefs. The astral plane comprises the infinite "astral sea" plus within it its starry "wildspaces" and conceptual "dominions". A dominion is a realm of a specific concept, such as an ethical ideal, a cosmic library, a belief system, a dream, or a cultural construct.
The 1e outer planes are 5e astral dominions. Specifically these are astral dominions where thought encounters the concept of an ethical alignment, such as Chaotic Good or True Neutral. Perceptually, an astral sea traveler might seem like discovering a floating mote and entering a building on top of it. But actually, the thoughts are shifting to collective memories relating to an ethical concept.
Each alignment dominion has a creature subtype, analogous to an angel, that personifies and enforces the beliefs, dreams, and cultural structures that relate to its specific ethical alignment. A Lawful Neutral angel, sotospeak, is called a modron.
The inhabitants of an alignment dominion speak an alignment language, such as the Lawful Good alignment, which is also known as Celestial.
The rest of this post is a lengthy discussion for why the matrix above is the way it is. It derives from 1e but adjust to the current terminology of 5e. Sometimes it borrows from 2e, 3e, 4e, or other consideration to fill in a gap or to clarify a concept.
It is important that the Greyhawk setting is the one that belongs to Gygax, in other words: 1e only. Heh, but to be candid, 1e is sometimes a "formative" adhoc mess, and the later editions can be useful when explaining 1e more systematically.
Regarding the names of the alignment languages, 5e already has Celestial (dejure G but defacto LG), Infernal (LE), and Abyssal (CE). Primordial (NN) covers the cosmic neutrality language.
At this point, there are several ways to fill in the gaps in 5e languages. A more completist approach follows.
Regarding the Fiend creature type, the 5e Monster Manual mentions two subtypes, LE devil and CE demon, and Mordenkeinens Tome of Foes mentions the subtype, NE yugoloth. All three are Fiends.
Personally I strongly associate the Aberration type to be "the" NE Fiend. It is neither Lawful, seductive corruption nor Chaotic, insane selfishness. Any method toward the annihilation of the multiverse is acceptable. The Aberration is neutrally Evil. In this sense, yugoloth is a kind of Aberration, and the term Aberration might well be the NE subtype. For now, yugoloth stands as the NE creature subtype, but the 5e Deepspeech language of Aberration is the NE language.
5e calls the NE astral dominion "Hades". This name is problematic. The Greek concept of Hades is pretty much exactly what 5e calls "Shadowfell". Hades is defacto an alternate name for the 5e shadow plane. By contrast, the Greek concept of Tartaros is an aspect of the subterranean depth that is a place of torment and suffering. So, Tartaros is an appropriate name for a NE dominion, not Hades. The 1e Players Handbook uses the term Tartarus for the NE-E dominion, but 5e calls this Carceri. So Tartaros remains an appropriate name for the NE dominion. Tartaros with an -o- is the Greek name, while Tartarus with a -u- is the Latinized version of it. The 5e Tome of Foes mentions in Hades, Greek places like the River Styx, but perhaps these are better understood as Shadowfell portals to Tartaros and elsewhere. The other traditional name for the NE dominion is the "Gray Waste". This works well. It can especially connote the aftermath, after cosmic horror annihilates everything. In fact, the NE dominion, called Gray Waste, can be the same thing as the 5e Far Realms. A place of painful deconstruction. So, the name for the NE dominion works well as Waste. Its language is Deepspeech.
Note. The Evil fiendish subtypes by definition mean a specific alignment, namely LE devil, NE yugoloth, and CE demon. But individual Fiends can have a different alignment. Good demons can and do exist, but are rare. (In my campaigns, angels and devils have no free will, but simply mirror human behaviors. When a human culture does more Good activities, then corresponding angels become more powerful and influential, and make the material plane becomes a better place. When a human culture learns to harness their Evil impulses in ways that actually do altruistic Good for other people, then even the Fiends start to become Good, one by one.) (The angels and devils lack free will, but their avatars in the material plane, such as aasimar and tiefling gain free will, and are autonomous conscious beings, who may or may not choose to actionize their Celestial or Infernal higher self.) Obviously, a Good demon experiences tension and antagonism from the rest of the demons that enforce Chaotic Evil.
• LE devil in the Hells speaks Infernal
• NE yugoloth in the Waste speaks Deepspeech
• CE demon in the Abyss speaks Abyssal
Now to the Good factions.
Celestial, Elysian, and Liberan, serve as the Good alignment languages.
5e only has the Celestial language, which means LG, yet is confusingly intended for all Good beings within the astral dominions, including Chaotic Good and Neutral Good. Celestial is quintessentially a LG language. In my view, the term "celestial" itself connotes lawfulness, deriving from the celestial spheres and their constant and predictable cycle of seasons, along with their "celestial", benign cosmic order, the music of the spheres, and the mechanical gears of the universe. Even the corresponding LG astral dominion is called "Mount Celestia", making Celestial its LG language.
By contrast, the NG dominion is called Elysia. It derives from a Latinization of the Greek concept of "Elusion". It is an otherworldly place of altruistic pleasures. The Greeks explain its name as either coming from "aluo", meaning to cause extreme joy, or else from "alutos", meaning undissolvable, indestructable, irrefutable. Both explanations apply to the concept. It is the invincible joy of both enjoying the sources of pleasures oneself and learning how to share these pleasures with others. Elysia is Paradise, the Garden of Eden. As a NG language, Elysian works great.
In 5e the creature subtype "angel" can serve as the Neutral Good subtype. The reallife traditions and tropes about angels portray them as quirky, and while generally cooperative, they can be hugely disorderly if for the sake of arguing on behalf of a greater Good. Angels are not LG, certainly not all of them. They are "True Good", or Neutral Good. Similarly, in the 5e Monster Manual, the angel is a creature subtype that serves all Good astral dominions, whether LG, NG, or CG. While the 5e Monster Manual only mentions LG creatures for the angel subtype, namely deva, solar, and planatar, the 2e monstrous compendiums specifify that these are actually kinds of "aasimon", and likewise specifically, Lawful Good. For this reason, a Lawful Good analogue of an angel is more specifically called an aasimon. By contrast, the 5e creature subtype angel is better terminology for NG specifically.
Note. The 2e Monstrous Compendiums have names for alignment-oriented angelic beings. But all three names for the Good ones seem problematic for different reasons. The LG "archon" means something else in 4e. The NG "guardinal" actually feels like a better name for a Lawful Good influence, as "guarding" can connote conservatively defending against change, or protecting the members of a group. The Chaotic Good "eladrin" is now a fey elf. Therefore, the "aasimon" of the deva, solar, and planatar become perfect for a LG analogue of angel. The angel works great for NG, the angel proper.
This leads us to the CG analogue of an angel. The 2e Monstrous Compendiums called it an "eladrin". But 4e and now 5e have stolen the 2e subtype "eladrin", and made it mean a Fey elf. Which is fine. Because. The 2e eladrin is way too elfish and "reclusive" to function as a vastly cosmic faction anyway. I am unsure which creature subtype should function as the analogue for a CG version of an angel. But, there is wide consensus that the ethical ideal of the Chaotic Good alignment is: personal, individualistic, freedom. In the sense of Good, this is the impulse of an individualist to also help empower the individualism of others. To be CG is to set people free, so they can figure out the best versions of themselves. Each individual needs to figure out for oneself how to use ones own gifts to better the lives of other individuals. The Chaotic Good astral dominion is Arborea. But "arboreal" literally means "of a tree", and trees dont feel chaotic to me. If by tree, one really means "wood elves", then the term is way too specifically elven to function as a broader concept of a benign cosmic freedom. The name for the CG language works better if denoting "freedom" itself. I suggest, "Liberan", a neologistic blend of liberal, liberation, and perhaps libertine. In Latin, libero is a verb meaning, "I set free". Here, the suffix "-an" parallels its use for some other alignment languages. (Heh, I couldnt call the language Liberal, because classic liberalism is strongly individualistic and about defending the freedom of choice of others, but new school progressivism feels more like a Lawful Good ideal, with strong social obligations, equity, and a group-oriented sense of fairness.) Perhaps the CG angelic analogue is called an "advocate". The term connotes a champion who fights on behalf of an other individual. The name of the CG dominion goes by a new alternate name, "Libera", in place of the name Aborea, in order to emphasize a cosmic alignment faction of freedom, and to deemphasize elfiness. It is still ok to use Arborea as an alternate name, or a nickname.
• LG aasimon in Mount Celestia speaks Celestial
• NG angel in Elysia speaks Elysian
• CG advocate in Libera speaks Liberan
For the Neutral languages, we already have 5e Primordial for Neutral Neutral, also known as True Neutral. Its dominion is Outlands. In 2e, the NN Outlands seem to lack a counterpart to an angel. 4e has the primordials, who are cosmic beings of the Elemental Chaos. These tend toward Chaotic Evil, and actually relate to the demonic Abyss within the 4e Elemental Chaos. Nevertheless 5e uses the term Primordial as the language of the Elemental creature type. In 5e, it defacto means a NN alignment. Meanwhile, a "primordial" works well as a NN analogue to an angel. Generally, it makes sense to equate the 1e Outlands with the 5e Elemental Chaos. Elementals are the neutral building blocks of the material universe. Meanwhile, the map of 5e cosmology never mentions the Outlands, because the Inner Planes block out where it would be. But in 5e, the Elemental Chaos is the threshold between astral plane and the ethereal plane. It makes sense when the Elemental Chaos and the Outlands are in fact the same thing. 1e Outlands is the better name. Elemental "Chaos" is Neutral Neutral, and not at all Chaotic. So, the NN alignment dominion called Outlands, and is the place that primordials inhabit. Of course, they speak Primordial.
The LN language relates modrons as the angel-like creature subtype, and in this case, the name of the astral dominion works well. "Mechan" is the LN language name. The neologism "Mechan" is a backformation from mechanic and Mechanus.
Chaotic Neutral is about extreme personal choice, to a degree that hates any kind of group-oriented obligation. The alignment seems impossible for a social animal, such as a human, to experience or understand. D&D traditions often characterize CN as mathematical randomness, tho this is unsuitable for a ROLEPLAY game. CN can relate to the power to change oneself and ones circumstance. The ultimate ethic of Chaotic Neutral is personal Choice. The CN individual insists on always having a Choice: to choose to do something or to refuse to do something, or to choose to do something else entirely. Choice. The Latin term for "choice" is electio, relating to English terms like "elect" and "election". I suggest the name for the CN language be "Electan", the language of choices. The -an suffix is under the influence of Mechan, as if Mech-an. One can rename the CN dominion Electa or Electia, but Limbo still kinda works, in the sense of a dilemma between many viable choices. The 1e CN counterpart to an angel is a slaad. The D&D traditions about the slaad tend to be "Chaotic Stupid" in the sense of mathematical randomness, disruptive to gameplay generally, and its inhuman quality invariably comes across as if antihuman and defacto Evil. Instead, the slaad needs to be an agent that humans and others can call on to grant options and choices when in difficult oppressive situations. The options are Neutral and not necessarily altruistic or predatory. But the options are real, pragmatic, and viable. Even better to grant several good options to choose from. Perhaps the slaad shifts form, mercurially, one form for each option that it grants. The slaad is like a CN version of an angel.
• LN modron in Mechanus speaks Mechan
• NN primordial in Outlands speaks Primordial
• CN slaad in Limbo speaks Electan
In Summary.
Alignment (Astral Dominion, Creature Subtype): Language
Good dominions:
• Lawful Good (Mount Celestia, aasimon): Celestial
• True Good / Neutral Good (Elysium, angel): Elysian
• Chaotic Good (Libera aka Arborea, advocate): Liberan
Neutral dominions
• Lawful Neutral (Mechanus, modron): Mechan
• True Neutral / Neutral Neutral (Outlands, primordial): Primordial
• Chaotic Neutral (Electa aka Limbo, slaad): Electan
Evil dominions:
• Lawful Evil (Hells, devil): Infernal
• True Evil / Neutral Evil (Waste, yugoloth): Deepspeech
• Chaotic Evil (Abyss, demon): Abyssal
