locale.format_string doesn't return same result as a normal
"string" % format
directive, but raises a TypeError. See attached test case for Python
2.6.

>>> locale.format_string('%f%%', 1.0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/locale.py", line 195, in format_string
    return new_f % val
TypeError: not enough arguments for format string
>>> '%f%%' % 1.0
'1.000000%'