Nikola build traceback (just one date_format)

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Traceback (most recent call last):
  File "c:\python35\lib\site-packages\doit\doit_cmd.py", line 168, in run
    return command.parse_execute(args)
  File "c:\python35\lib\site-packages\doit\cmd_base.py", line 122, in parse_execute
    return self.execute(params, args)
  File "c:\python35\lib\site-packages\doit\cmd_base.py", line 380, in execute
    self, params, args)
  File "c:\python35\lib\site-packages\nikola\__main__.py", line 279, in load_tasks
    self.nikola.gen_tasks('render_site', "Task", 'Group of tasks to render the site.'))
  File "c:\python35\lib\site-packages\doit\loader.py", line 314, in generate_tasks
    for task_dict, x_doc in flat_generator(gen_result, gen_doc):
  File "c:\python35\lib\site-packages\doit\loader.py", line 28, in flat_generator
    for item in gen:
  File "c:\python35\lib\site-packages\nikola\nikola.py", line 1638, in gen_tasks
    for task in flatten(pluginInfo.plugin_object.gen_tasks()):
  File "c:\python35\lib\site-packages\nikola\nikola.py", line 1632, in flatten
    for t in task:
  File "c:\python35\lib\site-packages\nikola\plugins\task\archive.py", line 197, in gen_tasks
    year=year, month=nikola.utils.LocaleBorg().get_month_name(int(month), lang))
  File "c:\python35\lib\site-packages\nikola\utils.py", line 1186, in get_month_name
    s = calendar.month_name[month_no]
  File "c:\python35\lib\calendar.py", line 497, in __exit__
    _locale.setlocale(_locale.LC_TIME, self.oldlocale)
  File "conf.py", line 20, in fancy_setlocale
    sl(*args, **kwargs)
  File "c:\python35\lib\locale.py", line 595, in setlocale
    return _setlocale(category, locale)
locale.Error: unsupported locale setting