Django : translation not found only during tests

I'm doing functional tests (Selenium) on my django app, and i want to include some fixtures. So first, i tried to load fixtures using manage.py :

./manage.py dumpdata > db.json
./manage.py loaddata db.json

No errors, i decided to use this data for my tests :

class FrontTest(LiveServerTestCase):

    fixtures = ['db.json']

But when i run my test :

django.core.serializers.base.DeserializationError: Problem installing fixture '/srv/app/event.json': EventBlock has no field named 'content_fr'

Have you ever get this error ?

Thank you in advance.