Add en back to all locale options

The option was not being displayed because the en.yml was removed from
this project

Also add another couple of tweaks to fix the build
Account for rails default locale on initialization

Users setting rails i18n.default_locale were getting errors because
spree_i18n didn't change I18n.locale accordingly and globalize3 depends
on I18n.locale to set the language when creating a translated record

Fixes #248