According to https://github.com/osTicket/osTicket-1.8/tree/develop/include/i18nStarting a new translation
Copy the entire en_US folder and all its contents and subfolders to a new
language code, such as de_DE. This is recommended as opposed to copying the
contents of another translation, because the en_US folder is the most
up-to-date in terms of content. osTicket is developed with en_US as the
primary language. Therefore new features will be implemented against the
en_US data first. If you are beginning a new translation, start with the
most current texts.You can download language packs from:
http://osticket.com/download-edge#linguas
This feature is not fully implemented at this time. Please read: http://www.forum.osticket.com/d/discussion//hello-habari-hallo-bonjour-hola-ciao#latest
and
http://www.forum.osticket.com/d/discussion//osticket-v1-8-0-2-and-v1-7-5-released#latest