The mod RESOURCE and PROGRAM scripts determine what languages are are listed and supported by the mod.
The Config.exe dropdown list for the mod is populated from a section added to the RESOURCE\INI\texts\language.ini. Add a list of supported languages to the file, like this:
[ConfigList]
cfgLang = English
cfgLang = French
cfgLang = German
cfgLang = Italian
cfgLang = Spanish
If you want multiple languages (i.e. more than one language), then move your files for each language into separate directories with the the name of the language as the subdirectory name, and in your Program script code include files, you can use the special $language preprocessor in the file path and the user's selected language will replace that path with their current language. The Maelstrom engine will replace all instances of $language in the code script with the selected language during the game startup while compiling the game scripts. Example from Program\DIALOGS\portuguese\Capitans_dialog.c and Program\DIALOGS\english\Capitans_dialog.c
#include "DIALOGS\$language\Rumours\Common_rumours.c"
#include "TEXT\$language\DIALOGS\Capitans_dialog.h"
Of course the additional work for the font.ini files, and font.tga textures must also be set up for each language, but that is also the mod responsibility.