Translations file are .xml which have all the translations strings need to have Notepad X in your language.

To change your language open the Options dialog from Tools menu located at Main Menu.
To download translations use the build-in download manager located at Options dialog or you can download them manually from here and put them at Notepad X directory\Translations

To create translations you can use the Translations editor plugin or edit manually this empty translation file and then submit it at Translations Submit Page.

For any questions about translating Notepad X use our Translations Forum
Some translations strings may change in the future check this post for any changes