All language files will be bundled with the jar on downloading. Simply extract the compressed folder into your plugins folder and everything should work fine. The initial set of languages have been translated using Google Translate, and as such may not be entirely accurate.
If you would like to translate another language or correct one of the existing translations, paste it to bukkit and post a link below. The name of the paste should be "RankDisplayNames - " followed by the filename.
See LanguageAPI for details on how to translate and name these files.
The default language file: lang-eng.yml