🌐(backend) user language from request Content-Language

We want to adapt the email language depend the website
choosen language. We get the website language
from the request Content-Language header.
We adapt the serializer to set the user language
from the request Content-Language header.
Thanks to that our email will be in the right language.

src/backend/core/api/serializers.py

@@ -262,6 +262,7 @@ class InvitationSerializer(serializers.ModelSerializer):
                 "Only owners of a document can invite other users as owners."
             )
 
+        user.language = request.headers.get("Content-Language", "en")
         attrs["document_id"] = document_id
         attrs["issuer"] = user
         return attrs