Add users to static language group

When setting language in extra's put them in static group too. So we can use Group Router to send them to correct dialogue for Service Rating.
[`groups.push()`](http://vumi-jssandbox-toolkit.readthedocs.org/en/latest/contacts.html#Contact) expects a `group.key`, not sure what you're expecting this will do because `en` isn't a group key.