In order to use your own custom language, you need to change the file called language.yml inside the folder SmartCore. You can also check for premade languages on the sidebar.
Remember, when you translate, whatever you do don't change the key.
Example, if you want to translate the line:
noPermission: '%error-prefix% &cYou do not have permission to execute this command!'
you can only change the value, and not the key. The value is the text that is inside the string (' '). You can't change the key.
Let me explain why.
When a player don't have permission to a command, the plugin is searching for the key noPermission. If the key was change to something else, like permissionNo, then the plugin can't fint the key noPermission.