The feature page has much useful information (I think!)
The wordpress date_i18n function requires that the base wordpress translation files be loaded. The wp-native-dashboard plugin is useful to help you download these. Note
- different languages store the latest files in different places!
- the native dashboard works for your current version of WordPress. If the language file as not been updated, then it may not show up. An older version may still be fine for your purposes.
Other posts that may be helpful: