You can check this post Localisation documentation
And also check the Language.h file found in every mod language, where specific functions are declared to cope with language necessities.
The english one it’s in \Program files\Steam\steamapps\common\Ancient Cities\Ancient\Data\Ancient\Language\en\Language.h
The other ones are inside the mod internal directory \Program files\Steam\steamapps\common\Ancient Cities\Ancient\Data\Ancient\Mod[Language Mod]\Ancient\Language\en\Language.h
We update the How to create a language mod