Ok i see. So you think it's possible to edit the description of the NPCs? And add small details about, for example, the language they uses or other details about the lore that i want to establish? Because as you say, using the dialog window to prompt the AI about the NPCs works, it's true, but it can be unstable and especially be forgotten after a while.
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.