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.
You can find more information on our Translation Working Group at the main Joomla! site, including a list of Translation Teams Coordinators and a description of what the team on a whole is responsible for. http://dev.joomla.org/content/view/22/53/ (This listing is rather obsolete at this present time.)
For practical informations on the way to make core Translation files, please see forum: http://forum.joomla.org/viewtopic.php?f=11&t=254709