Update to phpBB 3.1.0-b3-dev (Build 1399).

https://bamboo.phpbb.com/browse/PHPBB3-DEVELOP-1399
Some language changes.

Some words has been changed in accordance with the recommendations of
the Délégation Générale à la Langue Française et aux Langues de France.
That's why "template" is now translated to "modèle" and "plugin" to
"module d'extension".
Increase the version to 2.0.0-dev (Build 70).