Here are some modules, posts and discussion on how to do multiple languages in Drupal 6:

• Updating translations for Drupal 6
• Tutorial - Building a multi-language site
• How to install a different language
• Language negotiation - domain only
• How to translate Menu items
• Translating Menus in Drupal 6
• Module Internationalization
• Module Virtual Sites 
• Handling missing translations
• I did some modification to deal with the duplicate url problem