Add language switcher to main navigation

This adds the i18n urls to the app.py and adds a language switcher partial to layout.html

Please note that the following changes have to be made to the project specific settings file:

1. All languages that are to be added to the language switcher must be defined in the LANGUAGES setting, including English.
2. 'django.core.context_processors.i18n' must be added to the TEMPLATE_CONTEXT_PROCESSORS setting.
3. 'django.middleware.locale.LocaleMiddleware' must be added to the MIDDLEWARE_CLASSES setting.