Django - ValueError: plural forms expression could be dangerous

I'm working with Django 1.9.6 in Microsoft Visual Studio, using a virtual environment where all the python packages my project requires are installed. One of the characteristics of my project is internationalization, but it is giving me problems. I have created the translation messages and I have compiled them correctly, but when I launch the server the following error occurs:

Traceback (most recent call last):
File "C:\ruta-proyecto\env\lib\site-packages\django\core\handlers\base.py", line 123, in get_response
response = middleware_method(request)
File "C:\ruta-proyecto\env\lib\site-packages\django\middleware\locale.py", line 26, in process_request
translation.activate(language)
File "C:\ruta-proyecto\env\lib\site-packages\django\utils\translation\__init__.py", line 154, in activate
return _trans.activate(language)
File "C:\ruta-proyecto\env\lib\site-packages\django\utils\translation\trans_real.py", line 216, in activate
_active.value = translation(language)
File "C:\ruta-proyecto\env\lib\site-packages\django\utils\translation\trans_real.py", line 205, in translation
_translations[language] = DjangoTranslation(language)
File "C:\ruta-proyecto\env\lib\site-packages\django\utils\translation\trans_real.py", line 114, in __init__
self._add_installed_apps_translations()
File "C:\ruta-proyecto\env\lib\site-packages\django\utils\translation\trans_real.py", line 160, in _add_installed_apps_translations
translation = self._new_gnu_trans(localedir)
File "C:\ruta-proyecto\env\lib\site-packages\django\utils\translation\trans_real.py", line 140, in _new_gnu_trans
fallback=use_null_fallback)
File "C:\Python27\Lib\gettext.py", line 481, in translation
t = _translations.setdefault(key, class_(fp))
File "C:\Python27\Lib\gettext.py", line 182, in __init__
self._parse(fp)
File "C:\Python27\Lib\gettext.py", line 318, in _parse
self.plural = c2py(plural)
File "C:\Python27\Lib\gettext.py", line 95, in c2py
raise ValueError, 'plural forms expression could be dangerous'
ValueError: plural forms expression could be dangerous

[26/May/2016 11:24:06] "GET / HTTP/1.1" 500 104014
El subproceso 0x1834 terminó con código 0 (0x0).
Internal Server Error: /favicon.ico
Traceback (most recent call last):
File "C:\ruta-proyecto\env\lib\site-packages\django\core\handlers\base.py", line 123, in get_response
response = middleware_method(request)
File "C:\ruta-proyecto\env\lib\site-packages\django\middleware\locale.py", line 26, in process_request
translation.activate(language)
File "C:\ruta-proyecto\env\lib\site-packages\django\utils\translation\__init__.py", line 154, in activate
return _trans.activate(language)
File "C:\ruta-proyecto\env\lib\site-packages\django\utils\translation\trans_real.py", line 216, in activate
_active.value = translation(language)
File "C:\ruta-proyecto\env\lib\site-packages\django\utils\translation\trans_real.py", line 205, in translation
_translations[language] = DjangoTranslation(language)
File "C:\ruta-proyecto\env\lib\site-packages\django\utils\translation\trans_real.py", line 114, in __init__
self._add_installed_apps_translations()
File "C:\ruta-proyecto\env\lib\site-packages\django\utils\translation\trans_real.py", line 160, in _add_installed_apps_translations
translation = self._new_gnu_trans(localedir)
File "C:\ruta-proyecto\env\lib\site-packages\django\utils\translation\trans_real.py", line 140, in _new_gnu_trans
fallback=use_null_fallback)
File "C:\Python27\Lib\gettext.py", line 481, in translation
t = _translations.setdefault(key, class_(fp))
File "C:\Python27\Lib\gettext.py", line 182, in __init__
self._parse(fp)
File "C:\Python27\Lib\gettext.py", line 318, in _parse
self.plural = c2py(plural)
File "C:\Python27\Lib\gettext.py", line 95, in c2py
raise ValueError, 'plural forms expression could be dangerous'
ValueError: plural forms expression could be dangerous

However, if I launch the server from the command line using

python manage.py runserver

no error occurs. I have read other posts about this error ( link and link ), but the proposed solutions have no effect.

Update 3: The settings.py is as follows:

from django.utils.translation import ugettext_lazy as _

# Build paths inside the project like this: os.path.join(BASE_DIR, ...)
import os

BASE_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))

# SECURITY WARNING: keep the secret key used in production secret!
SECRET_KEY = #secret-key

# SECURITY WARNING: don't run with debug turned on in production!
DEBUG = True

ALLOWED_HOSTS = (
    '*',
)
APPEND_SLASH=False

INSTALLED_APPS = (
    'django.contrib.admin',
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    'rest_framework',
    'app1',
    'app2',
)


MIDDLEWARE_CLASSES = (
    'django.contrib.sessions.middleware.SessionMiddleware',
    'django.middleware.locale.LocaleMiddleware',
    'django.middleware.common.CommonMiddleware',
    'django.middleware.csrf.CsrfViewMiddleware',    
    'django.contrib.auth.middleware.AuthenticationMiddleware',
    'django.contrib.auth.middleware.SessionAuthenticationMiddleware',
    'django.contrib.messages.middleware.MessageMiddleware',
    'django.middleware.clickjacking.XFrameOptionsMiddleware',
    'django.middleware.security.SecurityMiddleware',
)

ROOT_URLCONF = 'webapp.urls'

TEMPLATES = [
    {
        'BACKEND': 'django.template.backends.django.DjangoTemplates',
        'DIRS': [os.path.join(BASE_DIR, 'templates')],
        'APP_DIRS': True,
        'OPTIONS': {
            'context_processors': [
                'django.template.context_processors.debug',
                'django.template.context_processors.request',
                'django.contrib.auth.context_processors.auth',
                'django.contrib.messages.context_processors.messages',
            ],
        },
    },
]

WSGI_APPLICATION = 'webapp.wsgi.application'


# Database

#database-settings    

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
        'LOCATION': '127.0.0.1:11211',
    }
}

CACHE_TIMEOUT = 60 * 15

# Internationalization

LANGUAGE_CODE = 'en'

LANGUAGE_CHOICES = (
    (1, "es"),
    (2, "en")
)
LANGUAGES = (
    ('es', _('Castellano')),
    ('en', _('English')),
)
TIME_ZONE = 'Europe/Berlin'

USE_I18N = True

USE_L10N = True

USE_TZ = True

LOCALE_PATHS = (
    os.path.join(BASE_DIR,'locale'),
)

# Static files (CSS, JavaScript, Images)

STATIC_ROOT = os.path.join(BASE_DIR,'static')
STATIC_URL = '/static/'

# Additional locations of static files
STATICFILES_DIRS = (
    # Put strings here, like "/home/html/static" or "C:/www/django/static".
    # Always use forward slashes, even on Windows.
    # Don't forget to use absolute paths, not relative paths.
    os.path.join(BASE_DIR,'assets'),
)

# List of finder classes that know how to find static files in
# various locations.
STATICFILES_FINDERS = (
    'django.contrib.staticfiles.finders.FileSystemFinder',
    'django.contrib.staticfiles.finders.AppDirectoriesFinder',
#   'django.contrib.staticfiles.finders.DefaultStorageFinder',
)

Is there a way to solve this problem?