Added sanity check regarding i18n behavior when setting the Config.language property

`$this->Session->read('Config.language)` returns `null` when the
property doesn't exist in the Session object, and so sets the
`Config.language` property to `null`, overriding the default setting
that was set by the bootstrap file, as suggested in the previous
paragraph.

Adding this simple sanity check ensures that no change is made to the
property if no locale information exists in the Session object.