If you browse to /admin/store/settings/checkout/edit/messages in your store, you should see "This is a multilingual variable." under each of the text entry boxes. This means that if you switch the interface language and save the message again, it will use that message in when users browse in that language. For example in my English/French stores, I would need to save a message in English, then switch to French and save a message again in French. Then when French users checkout Ubercart will automatically choose the French version I saved, and default to English.