First copy an existing language file, such as en.js from src/assets/js/trillek/i18n/. Rename and translate as you see fit.
Then open up src/assets/js/trillek/i18n/main.js and copy an existing config line, for example: .translations('en', require('./en')). Change the language code to point at your new file.
Now open up src/assets/js/trillek/config/default.js and add your new language code and translated label to the availableLanguages object. The full name should be written in the actual language it is for, so when it shows up in the selection list the user can actually find their language.