Bryan Ruby points out that many open source content management systems are started to think about multilanguage support as a core building block recently. Drupal 6 is one of these systems, and although it does not come with complete internationalization and translation features, it goes a long way compared to Drupal 5. Jose A. Reyero pulled together a nice comparision table of the Drupal 5 and 6 core multilanguage features. As his table shows, right to left (RTL, eg. Arabic, Hebrew) language support is improved considerably. Now we know about each post being written in an RTL language or not, and we know whether the language used to present the page is RTL. All is left is complete theme coverage, so themes can be RTL-aware. Drupal 6 comes with automatic discovery of RTL CSS files, so a theme can easily support RTL styles.
The basic core themes, such as Bluemarine, Marvin, and Chameleon already include RTL style sheets, but the three (actually two) bigger ones: Garland/Minnelli and Pushbutton lack RTL support. The efforts to bring RTL styles to Pushbutton are blocked on small CSS bugs. So if you care, please look into this issue: http://drupal.org/node/148084 There is also a garlandrtl theme released for Drupal 5, which has some custom hacks to recognize an RTL language, and otherwise needs to be cleaned up to get to Drupal 6 (as part of the Garland/Minnelli theme, not as a separate theme).
Help with these issues would be very welcome to have a complete RTL theme offering in Drupal 6!