Nobody knows when Valdyas was first settled, or who settled it. The people who live there now, however, clearly have a common ancestry. They all worship the same gods and speak the same language. Roughly the same language...
The language as it is spoken now - in the reign of King Athal, who came to the throne in 535 - is much more homogeneous than it was when the kingdom was founded. But it's not for nothing that the common name for the language as a whole is Ilaini, "the languages", pronounced [i'lani].
Even now, when a man from Tal-Crun in the west marries a woman from Rychie Gralen in the east and takes her home to meet his parents, chances are that they can't understand a word she's saying; neither do his in-laws understand him. Educated people can usually speak "the Queen's speech" as well as their own dialect. Children learn to speak it in village schools (though history and arithmetic may be taught in dialect) and any school beyond the elementary level uses it for all lessons.
People are proud of their dialect, and you're not really taken seriously if you can't "talk broad" even if that's a very different kind of "broad" from what is spoken where you happen to be. Usually, when travelling, you speak your own dialect at first to establish that you can, then use valein ilain ("the king's/queen's speech") to make yourself understood.
Obviously, if you happen to be from Valdis where valein ilain is the local dialect, you have a problem, and indeed people from Valdis have a hard time being taken as seriously as others when they're not at home.
During the reign of King Vegelin the Great (353-380) more and more was being written down, and it was clear that the various systems of writing people had been using didn't suffice any more. The King's head clerk and historian, Mailei Halla (as famous for the Book of Halla's Left Hand as for her more serious exploits), is said to have developed a script and rules for orthography all by herself, but it's likely to have been a joint effort of the court clerks that Halla had the last word in.
Halla is rightly credited with writing a history of Valdyas in three parts: everything known about the clans before the kingdom came into being, the early history of the kingdom up to Vegelin the Great's grandfather Meruvin, and records of the time of Vegelin's mother Queen Mialle and Vegelin himself. Many copies were made. The original of the first part has always been in the palace library, but the others were lost until the second part came into the possession of Queen Alyse I's treasurer, Selevei Aylin astin Brun, and the third part was retrieved from Dol-Rayen before it fell.
Because so many people can read books are in great demand; you can have a book copied in a galyas ("writing shop") or pay a smaller fee for the use of ink and a desk to do the work yourself. Simple block printing is in use, and broadsheets are another way to provide reading matter for those who can't afford books.
Ilaini is a highly inflected language with a basic SOV word order. It's currently in a state of transition from prepositions to postpositions and from prefixes to suffixes: when two versions of a particular phrase exist, one with a preposition and one with a postposition, the prepositional phrase is usually the older. All nominal suffixes are still productive, and only some of the nominal prefixes.
Borrowings from other languages are usually made to conform to Ilaini patterns; calesh (a poisonous herb) is one of the very few exceptions.