The aspects of Joomla internationalization are covered in the following three specifications:

The localization of the entire Joomla application is decribed here. Guidlines for new language packages and 3PD extensions is also included. Joomla Localization API Specification

The impementation of utf-8 in Joomla is described here and guidlines for 3PD’s are given. Joomla UTF-8 Specification

This specification covers the methods that Joomla handles RTL compatibility and provides guidlines to 3PD’s for creating RTL aware components and templates. Joomla RTL ( Right to Left ) Specification