django locale不能依赖于accept语言

Question

这是我的设置

LANGUAGES = (
    ('zh_CN', u'简体中文'),
    ('en-us', u'English'),
)
LANGUAGE_CODE = 'en-us'

USE_I18N = True

USE_L10N = True

MIDDLEWARE_CLASSES = (

    'django.contrib.sessions.middleware.SessionMiddleware',
    'django.middleware.locale.LocaleMiddleware',
    'corsheaders.middleware.CorsMiddleware',
    'django.middleware.common.CommonMiddleware',
    'django.middleware.csrf.CsrfViewMiddleware',
    'django.contrib.auth.middleware.AuthenticationMiddleware',
    'django.contrib.messages.middleware.MessageMiddleware',
    # "django.middleware.cache.CacheMiddleware",
    "django.middleware.transaction.TransactionMiddleware",

)

这是一些打印

('django_language', '')
('COOKIES', '')
('ACCEPT_LANGUAGE', 'zh_CN')
('LANGUAGE_CODE', 'en-us')

我的django版本为1.5.4

如果我设置

request.session['django_language'] = 'zh_CN'

下次我会得到翻译。怎么了？