Because this game is use built-in font.
..\Mount & Blade II Bannerlord\GUI\GauntletUI
In this folder, file "Languages.xml", you need insert your language data and set font.
..\Mount & Blade II Bannerlord\GUI\GauntletUI\SpriteParts
In this folder have some png file for font.
Hmm...
Now, I have no tools to create fnt file.