How to add a new language to KCD's setting options?

I am localizing this game, and it is almost complete. But I have an idea, and I don’t know how to implement it: the way this localized mod works so far is to directly replace one of the languages in the game. But I now hope that my localized language can be independent as a single.

I mean, I want it to be added to the settings options and switch to my localized mod by selecting the language. So if I want to implement it, what should I do?

I only know that the language selection project is in text_ui_menus.xml, and I’m pretty sure that just adding it here doesn’t make it work, I need to know which other documents I should edit.