Here I'll write here all information I can get about Illageralt.
If you read "The Rise of the Arch-Illager", you understand, Villagers and Illagers are speaking on the same language.
But they are enemies to each other. That means, they won't call it with the same name.
Illagers called it Illageralt. Villagers called it Villagerese.
I made a photo to see all runes of this language:
Here it is



It works like english alphabet but there are other symbols.