fallback
You could try creating a Bugzilla request for an enhancement, that enables each user to choose their fallback language in the preferences, right next to where the preferred language is chosen right now.
But give some thought to the fact that translating the messages here (not locally, because that helps no one) is the best immediate way of ending the issue once and for all. A translated message always prevents the fallback from being presented, and that's the end of that. And it's in your hands, so you have the power to change things this way.
I know, I know... I must find some better way to show my protest ;)
Has anyone created a Bugzilla about this, per Hamilton Abreu's suggestion? If not, I will. This would at least help those readers who have user accounts. A good example of a language where this would be useful is Luo (OK it doesn't have a project here yet, but some day...). This language group lives either side of the border between Kenya and Tanzania. It would be useful to them to be able to choose either English (medium of primary instruction in Kenya) or Swahili (medium of primary instruction in Tanzania) as fallback.
Great. I have added my vote to the bug - my first vote ever on Bugzilla. I would say that this was very exciting, except that might give the impression that I don't get out enough :).
Thanks