I haven't paid much attention to the localization issues in `argparse`.  

The issue is with the help modification done by the:

class ArgumentDefaultsHelpFormatter

    help += ' (default: %(default)s)'

This formatter is a convenience, not something critical.  The programmer could always include that 'default' string in the help line with their preferred terminology.  They don't have to use this formatter to show the default values in their help lines.

_() is used mostly for error messages with the format 

    msg = _('message %(value)s')
    raise Error( msg % args)

So the following change might work:

    help += _(' (default: %(default)s)')

But I'm not in a position to test it or evaluate its usefulness.