The incompatibility worries me a bit.  A program compatible with Python 3.1 and 3.2 would have to support both sets of messages; I don’t know whether the i18n tools support having all the strings in one PO file, and how inconvenient it would be for translators.  I assume such transitions have happened in many projects and that people just accept the inconvenience.

Attached patch changes the strings to avoid warnings.  No tests added (I intend to write tests for i18n in argparse in the future, see #10497 and #10529).

Steven, I’d like your +1 before committing anything.  One message in particular is not very helpful, but I’ve been unable to find better placeholders (it’s in ArgumentPaser._get_value):
  msg = _('invalid %(name)s value: %(arg_string)r')