Force locale to :en for worldwide root

Users browsing back to the root of Worldwide via the main navigation
should always see it in the English locale. This is primarily to stop
the layout being right-to-left when browsing back from a page
translated in a right-to-left language.
Merge pull request #388 from alphagov/force-locale-for-worldwide-root

Force locale to :en for worldwide root

https://www.pivotaltracker.com/story/show/46807663