Send notifications using the system language as a fallback

[diegobz]

Notifications currently tend to be sent using the language of the user which is doing the action, in case the get_notification_language function is not able to identify the language of the target (receiver) user.

For a international audience this is not very nice, having in mind that, if I'm a user that speaks Brazilian Portuguese, I might be sending localized (pt_BR) notifications to people who do not speak it.

The following patch makes the notification system to use the default language set in the Django settings, in case no language is found for the target user.

https://gist.github.com/904552