You will probably also want to look at the Language Icons module, which provides the flags. Some details about this module (from its project page):
This module provides icons for language links, both for the Language switcher block and (optionally) for node links. It is a spin-off from Internationalization (i18n) package.
As basic multilingual support is now built into Drupal 6 core, it doesn't depend anymore on any other contributed module. This module will work as simple add-in for Drupal 6.
However, for a fully enabled multilingual site, the Internationalization package is recommended.
Setup is very important with localization. I suggest that you start -- preferably on a test site -- by setting Language negotiation to "Path prefix only" and Content selection mode to "Mixed current language (if available) or default language (if not) and language neutral"