One of the daffier ideas from 1st edition AD&D that people sometimes bring up is the concept of alignment languages. The rules stated that people of the same alignment actually had a special common language that they automatically knew, and only people of their alignment could speak. Apparently Gary Gygax originally intended it to be something like a ceremonial language used by religions, like Latin for Catholicism or Hebrew for Judaism, but as written it came across as a full language that anyone of a given alignment knew and somehow no one else could learn. The general consensus was this was Very Silly and they promptly disappeared in 2nd Edition, I'm pretty sure never to be seen again. In fact as each new edition of D&D comes out they seem to push alignment further and further into the background. You get the sense that a lot of people find it a bit embarrassing, when they don't ignore it entirely.
While alignment languages are odd, when you imagine people being able to speak something purely due to their moral outlook, I think there's an argument to them making sense in the classic D&D universe. After all, it's a world where alignments are a Real Thing. It's not just a person's moral code. It's an actual fundamental building block of the cosmos, that you can verify with magic. If certain magic items only work for people of specific alignments, why not languages?
A lot of fantasy settings have a Dark Speech, Demonic, Abyssal etc that evil creatures all use. The language itself isn't evil though (though invariably described as 'guttural', 'harsh' etc), it's just like a lingua franca for meanies. Lingua wanka?
I want to have alignments in my Dodex game, though a more simplified version to D&D. I'm going for Order, Chaos, Light and Dark. There's no Neutral, that's simply an absence of alignment. Most animals don't have an alignment, and not all people do either. It's assumed that player characters all do because they're Important People who have the potential for dire fates and grand destinies (even if half the time they end up dead in a pit trap). People are kind of aware of alignments in-game, they're seen as innate principles of life. Positive and negative forces, structure and entropy, like yin and yang. Depending on someone's belief system they may consider them to be literal pillars of existence, like gravity or magnetism, other people might see them more as metaphorical symbols for how things are.
Anyway I want to have alignment languages in Dodex! The common trade language in the game is called Tongue, which is accompanied by a physical sign language called Shape. There's also a secret(ish) one called Cant that's used by dodgy types, like a parallel trade language for the shadow economy. But then there's the alignment languages too. They're a type of magical communication, in a way, a bit like the one wizards use to scribble in their spellbooks. Something about the way they're constructed just feels wrong to people whose essence doesn't vibrate at that particular pitch. People of different alignments can still learn the language, but they struggle with it. Given time and effort they can translate things, but they'll never be truly fluent. As a result most people don't bother trying. In fact most people don't learn them anyway, it's not any easier to learn than any other language even if it is your alignment.
Aria is the language of Light. It is a tonal language where sentences rise and fall melodically, building to crescendos of meaning. It is not subtle... you can't whisper in Aria. It's also difficult to be sarcastic or insincere. You can be angry, or melancholy, or deceitful even, but the language forces you to do it in a declarative and heartfelt way. In its written form Aria looks like branches of swooping musical notation radiating out from a central point.
Ergo is the language of Order. Speakers sound like they're reading out a long set of equations, because essentially they are. Written down it looks like one of those giant chalkboards of sums you see in a film about mathematicians. Ergo is great at clarity, it's good for logistics. It is not good at nuance.
Gibberish is the language of Chaos. To non-speakers it sounds like a totally random series of made-up words and noises. They'll throw in a few barks and whistles too. No two speakers of Gibberish sound the same, but somehow they can always understand each other. There is no written form of the language.
Malison is the language of Dark. People expecting a grating, ugly form of speech are surprised at how pretty it can be. Its sing-song quality lends it to poetry and performance. But the flowery language hides its thorns. Malison has endless clauses and tenses, it twists on the tongue. It's impossible to pay a compliment without sounding snide, to speak of ideals without subverting them. It forces you to hold things back even when you try to tell the truth. The written form of Malison is like a palimpsest, patterns of sharp runic figures layered on top of each other.