See issue 10466 for background, but in short LocaleHTMLCalendar uses getdefaultlocale if no locale is specified, and on windows this results in a locale that setlocale will not accept.  The fix is presumably to use setlocale(LC_DATE, '') instead of getdefaultelocale.

See also #10087.

Fails on OS X as well for similar reasons.

On OSX, _locale.setlocale raise locale.Error with arg 'UTF8' but ok with 'UTF-8'. and setlocale tries to normalize localename with locale.normalize(). and locale.normalize() always change encoding to 'UTF8' (locale.py:646). 

So changes in locale.normalize could fix this issue at least on OSX. 

patch and unittest against p3k is attached.



I did not search why normalize() changes 'utf_8' to 'UTF8' until now. but I would post if I get answer in subversion log.

r38027 has following changes.

-    'utf-8':                        'UTF-8',
+    'utf_8':                        'UTF8',


subversion log is:

Correct mapping of Python codec name to C encoding name for UTF-8 (the C lib doesn't seem to like "UTF-8").

Your analysis is correct.  Sorry, I should have noted that the OS X "UTF8" vs "UTF-8" discrepancy has already been discussed in Issue10090 and Issue10154.