@remaye, you are right that this is indeed broken in Drupal/Ubercart. When I update the status of an order using the admin screen, the email will be sent out using the language that I, as the admin, am currently using. Ubercart doesn't care about the user's preferred language.

The only way around this I have found is for me to look up a user's preferred language manually, and then make sure I am viewing his order in the order admin screen using that language. It's a real pain.

I hate to say it but Drupal/Ubercart still hasn't gotten translations quite right.