Add a way to switch language in the UI

Celutz is now fully internationalized (French, English, Spanish). For now, it auto-detects the language based on the browser preference, and you can also force the language in the URL.

It would be nice to add a language selector in the interface, probably in the top bar. Something like this: https://docs.djangoproject.com/en/1.10/topics/i18n/translation/#miscellaneous