Adding language icons
In Drupal, language flags can be easily included in the language switcher with the Language Icons module http://drupal.org/project/languageicons.
Go to http://drupal.org/project/languageicons and download the module and install in your sites/all/modules directory.
Navigate to the module admisistration page in your website, /admin/build/modules, and enable the "Language Icons" module listed under the "Multilanguage" section.
Then save the configuration and that's it, you should now have flags in your language switcher.