All messages are stored in the MySQL database.
First start with this line (correct the values at the end) to add another language (example for Italian):
INSERT INTO `jts3servermodwebui_language` (`id`, `name`, `image`, `showLanguage`, `dateCountry`, `dateFormat`) VALUES(3, 'Italiano', 'IT.png', 1, 'it_IT', '%d.%m.%Y %H:%M:%S');
After you did this, every translation in the table jts3servermodwebui_translation needs the language_id = 3 (same id as in the table jts3servermodwebui_language).
In the table jts3servermodwebui_language you can also change the date format for a language (column dateFormat), or just hide this language from the language chooser at the top header of the web interface (column showLanguage).
In the far future I plan to implement a web interface to create a translation for the web interface. But currently this is the only way.
If you want, you can provide an SQL export of the new language for other users in this forum, post it in this board: https://www.stefan1200.de/forum/index.php?board=11.0