To change existing translations you would either make a custom language pack yourself (To do this you would copy the lanugage phar, extract the phar using PHP, and then edit the strings you want to edit. You can either leave it un-phared or re-phar it afterward. Then add it to osTicket.)

or

You can suggest alternative translations at the translation project located at http://i18n.osticket.com/. If the community accepts the translations they would become available the next time that the language pack is published.