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Administrators can change the current list of available locales with a Setting (formerly Global Property from platform 1.9 upwardsdownwards): locale.allowed.list
Go to Administration > Maintenance > Manage Global Properties
Use the two letter ISO-639-1 language code (see http://en.wikipedia.org/wiki/List_of_ISO_639-1_codes for a list of codes)
This Setting modifies Setting (formerly Global Property from 1.9 downwards) modifies the list of locales at the bottom of the page and the languages in the concept dictionary. The list of possible locale choices on the bottom of the page are modified with the locale.allowed.list, but only if there is a messages_xx.properties file to match the locale. In order to show the OpenMRS pages in Haitian Kreyol, complete these 2 steps:
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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 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.