j0rd is mostly correct.
Ubercart and E-Commerce are two separate projects, and "Drupal Commerce" is a third.
Drupal Commerce is a separate project started by ONE former Ubercart developer. Despite what others may falsely lead you to believe, Drupal Commerce is NOT Ubercart.
Ubercart 3.x started off as a straight port of Ubercart to Drupal 7, but some improvements have been included while waiting for Drupal 7 to be released and more will be included as long as we are waiting on Drupal 7.
As soon as Drupal 7 releases, we will release Ubercart 3.x. Then we will start working on 4.x which will include more improvements. The main reason for the quick change from major release 3 to 4 is that some improvements, like decoupling products from nodes, will break dependent modules and might have kept us from releasing 3.x in sync with Drupal 7.
We are interested in improving all aspects of Ubercart including the i18n support. You can submit patches and suggestions for improvements in the Ubercart issue queue. At this point I can't guarantee that they will make it into 3.x (we will try) but they will definitely make it into 4.x.
Cheers,
Andy
Ubercart Founder