Oscar is slowly adding better support for switching languages on the
fly. That also means that locales need to be switched.

The issue this commit tries to fix is that when switching languages, the
locale for rendering prices doesn't change, and hence prices are
formatted incorrectly.
Oscar used to require hardcoding the locale for prices via
OSCAR_CURRENCY_LOCALE. I'm not sure what the reasoning behind this was,
because as far as I'm aware, the locale can be auto-detected from the
current language. This not only reduces complexity, but nicely works
when switching languages.