A small distribution to install Drupal with user interface translations pulled automatically from localize.drupal.org. It is suggested you use this distribution to start off with Drupal if you'd like to set up a localized site. You can find setup instructions for the Drupal 7 version (which applies to Drupal 6 as well) at http://localize.drupal.org/node/2024
Localized Drupal ships with Localization update for sustainable translation management and Localization client for fixing translations and contributing back to the community (both on Drupal 6 and Drupal 7).
Look at http://drupal.org/project/issues/search?issue_tags=localized+install for the outstanding issues around this workflow, completing which should lead us to an ideal world. There is work to do.
Sponsored in part by Acquia.