Maybe your setting at file "Languages.xml" is like this:
********************
<Language id=" THAI FONT " DefaultFont="Galahad">
<Map From="FiraSansExtraCondensed-Light" To="FiraSansExtraCondensed-Light"/>
<Map From="FiraSansExtraCondensed-Medium" To="FiraSansExtraCondensed-Medium"/>
<Map From="FiraSansExtraCondensed-Regular" To="FiraSansExtraCondensed-Regular"/>
<Map From="Galahad" To="Galahad"/>
<Map From="Galahad_Numbers_Bold" To="Galahad_Numbers_Bold"/>
</Language>
********************
Language id is Thai font text.
DefaultFont="" is Galahad or other game fonts.
Font "Galahad" is not Thai font, so cannot display some letters in game.
********************
File Path:
..\Mount & Blade II Bannerlord\GUI\GauntletUI\Languages.xml
..\Mount & Blade II Bannerlord\GUI\GauntletUI\SpriteParts\ui_fonts\Galahad.png
..\Mount & Blade II Bannerlord\GUI\GauntletUI\SpriteParts\ui_fonts\FiraSansExtraCondensed-Light.png
..\Mount & Blade II Bannerlord\GUI\GauntletUI\SpriteParts\ui_fonts\FiraSansExtraCondensed-Medium.png
..\Mount & Blade II Bannerlord\GUI\GauntletUI\SpriteParts\ui_fonts\FiraSansExtraCondensed-Regular.png
..\Mount & Blade II Bannerlord\GUI\GauntletUI\SpriteParts\ui_fonts\Galahad_Numbers_Bold.png
********************
In Mod Language XML file:
..\Mount & Blade II Bannerlord\Modules\Native\ModuleData\Languages
All XML Header have this script.
<tag language="THAI FONT" />
This tag language need same "Languages.xml" Language id. Let game to grab the letters.
If your Thai Font is not in font file, you cannot show Thai font into the game.