The beginnings of the new language subsystem is in Drupal 6.x-dev!
Following on a huge amount of discussion and planning at the Internationalization Group, Jose A. Reyero, Károly Négyesi (chx) and Gábor Hojtsy (myself) prepared a big patch to introduce better language support into Drupal 6.x-dev. We got significant code reviews and correction tips from Dries Buytaert, Steven Wittens and Doug Green. As a result, the development version of Drupal 6 now includes a generic language setup screen with support for right-to-left written scripts, native language names, path and domain based language selection and browser language detection. We also added support for language dependent path aliases, so you can have addresses like "example.com/espanol/contacto" and "example.com/english/contact", or alternatively "english.example.com/contact" and "espanol.example.com/contacto".
Although some of this functionality was available in contributed modules, such as i18n module and localizer module, we plan on making Drupal 6 the first release to include decent language support at the core level, of which this was the first significant step. We tried to test as many aspects of the changes as possible, but still we would welcome more testers for this particular feature, as well as opinions and contributions for new functionality implemented for Drupal 6.
Join us in the Internationalization Group and grab our code from our Subversion repository (information on the group homepage), or test the changes already included in Drupal 6.x-dev, by downloading the latest development tarball.