Use LANGUAGE_CODE provided by Django's i18n

In `base.html`, the `lang` variable always shows `en-gb` even though language/locale settings are set in the config.

This fix only requires that `django.core.context_processors.i18n` is added to the `TEMPLATE_CONTEXT_PROCESSORS` setting (which I found was almost always set in the Oscar projects I've seen). If it isn't, then it defaults to `en-gb`.