--- a/cubicweb/server/session.py	Thu Nov 17 10:47:52 2016 +0100
+++ b/cubicweb/server/session.py	Thu Nov 17 10:53:04 2016 +0100
@@ -384,6 +384,8 @@
         assert not self._open
         self._open = True
         self.cnxset = self.repo._get_cnxset()
+        if self.lang is None:
+            self.set_language(self.user.prefered_language())
         return self
 
     def __exit__(self, exctype=None, excvalue=None, tb=None):