This is a mistake I made in commit f2ef27d3. Back then I thought we
needed to keep Globalize.locale and I18n.locale in sync, but the truth
is it automatically happens when setting Globalize.locale to nil.
So now we can use I18n.with_locale (at least in the tests) and forget
about Globalize, which will make it easier to switch to Mobility in the
future.