From Joomla! Documentation
As a translator you are asked to take on the responsibility of translating all the discrete strings found in the Joomla! front and back-ends as well as creating the necessary translation files. You do not have to be a Joomla! expert to contribute to the translation teams, and you do not have to have any development experience, although many of our translation workgroup members are experts and developers. You will need to know or learn how to use the translation debugging mode and how to assemble your translated strings into a translation file. You will also need to know or learn how to package translation files into an installable translation pack.
To see the available translation projects: http://community.joomla.org/translations/joomla-3-translations.html
Accredited 3.x translation packages: http://joomlacode.org/gf/project/jtranslation3_x/frs/