Update sandbox LANGUAGES to available languages in Transifex

I chose an arbitrary cut-off point of 50% coverage. The individual
entries are copied directly from Django's default settings.
Add support for i18n_patterns and enable for sandbox

This required:
* Moving Django's i18n urls into the main urls.py and outside of the
  scope of Oscar's main application class, because they need to live
  outside of the scope of i18n_patterns
* Add a language-code stripped version of the current URL to the
  template context to get the language switcher working.

I also updated the language switcher code to mimic what is currently
given as an example in Django's docs.

Fixes #797