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Drop a new messages_XX.properties file into the tomcathome/openmrs/WEB-INF/ to add additional translations. You will need to modify the Setting (formerly Global Property from 1.9 downwards) locale.allowed.list in order to choose the new language. This file will disappear after upgrades, so be sure to keep a backup.
Also see How To Translate the OpenMRS Platform and Modules for more detailed instructions.