Added multilingual support for the admin site

With Django's LocaleMiddleware it's very easy to add multilingual support. Here it's limited to the admin site.
You have to set USE_I18N = True and add your desired languages to LANGUAGES in settings.py

To do:
* Make the language selector only visible if USE_I18N is set to True

Something like {% if USE_I18N %} is required, but it doesn't work. Any ideas?

There is further one "problem", Django's LocaleMiddleware will add the language code as url prefix every time. If the main language is used, we don't want to add for example "/en/" as prefix. To prevent this, we can use django-solid-i18n-urls.

Here: https://github.com/st4lk/django-solid-i18n-urls/

That's just the admin interface, so don't worry about this.

With modeltranslations it's just one step away from full multilingual support.

Please try it and discuss about it! :)