iSymphony uses ISO 639-1 for language codes and ISO 3166-1 for region codes in the I18N system. These standards use two characters for the language code and two characters for the region code. For example, the default English language pack has a code of en-us. Using the ISO system is optional for custom language packs, however, if the locale does not match what the browser reports then you will need to manually specify the locale in the URL to use the language.
There are two restricted language/country combinations that will throw errors if used:
No language and no locale: client client-messges.properties
The en-us locale: client-messages_en-us.properties
Place the client files in the client servlet i18n folder: /opt/isymphony3/server/conf/client/i18n/
Place the admin files in the admin servlet i18n folder: /opt/isymphony3/server/conf/administrator/i18n/
The server will need to be restarted to load the new language files.