title: How to fix "plural forms could be dangerous" django error?
body: <p>I don't get it. If I set the <code>Accept-Language</code> header to anything other than "en" (de, pl, es) or even something that doesn't exist (like xxs), the app doesn't spit out this error, but it does when I set it to "en". It happens only on windows (newest gettext tools). Here's the stack trace:</p>
  
  <pre><code>Traceback (most recent call last):
    File "E:\projekty\python\myapp_api\_env\lib\site-packages\django\core\han
  dlers\exception.py", line 39, in inner
      response = get_response(request)
    File "E:\projekty\python\myapp_api\_env\lib\site-packages\django\core\han
  dlers\base.py", line 244, in _legacy_get_response
      response = middleware_method(request)
    File "E:\projekty\python\myapp_api\_env\lib\site-packages\django\middlewa
  re\locale.py", line 29, in process_request
      translation.activate(language)
    File "E:\projekty\python\myapp_api\_env\lib\site-packages\django\utils\tr
  anslation\__init__.py", line 161, in activate
      return _trans.activate(language)
    File "E:\projekty\python\myapp_api\_env\lib\site-packages\django\utils\tr
  anslation\trans_real.py", line 238, in activate
      _active.value = translation(language)
    File "E:\projekty\python\myapp_api\_env\lib\site-packages\django\utils\tr
  anslation\trans_real.py", line 227, in translation
      _translations[language] = DjangoTranslation(language)
    File "E:\projekty\python\myapp_api\_env\lib\site-packages\django\utils\tr
  anslation\trans_real.py", line 129, in __init__
      self._add_installed_apps_translations()
    File "E:\projekty\python\myapp_api\_env\lib\site-packages\django\utils\tr
  anslation\trans_real.py", line 176, in _add_installed_apps_translations
      translation = self._new_gnu_trans(localedir)
    File "E:\projekty\python\myapp_api\_env\lib\site-packages\django\utils\tr
  anslation\trans_real.py", line 156, in _new_gnu_trans
      fallback=use_null_fallback)
    File "C:\Python35\lib\gettext.py", line 426, in translation
      t = _translations.setdefault(key, class_(fp))
    File "C:\Python35\lib\gettext.py", line 162, in __init__
      self._parse(fp)
    File "C:\Python35\lib\gettext.py", line 297, in _parse
      self.plural = c2py(plural)
    File "C:\Python35\lib\gettext.py", line 76, in c2py
      raise ValueError('plural forms expression could be dangerous')
  ValueError: plural forms expression could be dangerous
  </code></pre>
  
  <p>I have <code>plural-forms</code> set up correctly in my <code>django.po</code> file:</p>
  
  <pre><code>"Plural-Forms: nplurals=2; plural=(n != 1);\n"
  </code></pre>
  
  <p>Why does this happen and how to fix it?</p>