This was proposed on python-ideas, discussed, approved, and implemented
for 2.7 and 3.1.

Note that although this wasn't discussed, the internationalization issue
is answered by the fact that when internationalizing, you are rewriting
the string.  At that time if you need a different order of the
subsitutions you can specify it, so requiring non-default numbering in
the original string doesn't really buy you anything for
internationalization.