That would require pulling apart the game files and modifying WorldLore.md which is not exposed like CharacterConfig.json is for custom characters. It's do-able, but could cause all kinds of things to go haywire depending on how picky the game is, and the model you use.
Just in the spirit of not crushing your hopes completely, Three Eyes had stated about a month ago that they plan on adding the ability to add custom NPCs at some point in the future, when they figure out how to best implement the feature. It may not be too big of a stretch that there may also be a way for them to make it so we can add our own modifiers to the game's base lore and characters, like changing the official language of Vyrndal, as you suggested. The final say is up to Three Eyes though.