I have made the Sinhala Language pack for Joomla 1.0.x and is availabe at www.joomlacode.org
Project Page: http://joomlacode.org/gf/project/sinhala/
Download the package at
http://joomlacode.org/gf/project/sinhala/frs/
Currently working on Sinhala package for Joomla 1.5