On most sites, you will get very far by having the interface in one single language and some information pages translated into several languages. But on some sites, this is not enough. The starting point for making truly multilingual sites is the Internationalization module. This module offers, among other things:

• Multilingual options for taxonomy terms and vocabularies, similar to those previously described for nodes.
• Multilingual options for menus.
• Translatable blocks, and block visibility settings based on language.

The project page for Internationalization contains a number of useful links for anyone who wants to read more about multilingual issues, and also lists more modules extending the multilingual capabilities of Drupal.