UI switch between languages broken when default locale is empty

You can see this on http://tuska.myxwiki.org/xwiki/bin/view/Main/Welcome (and probably most myxwiki.org wikis) which is supposed to have many languages and the Languages section we are supposed to get in the drawer does not appear.

It does not work for pages that were existing before the upgrade but it does work for new pages.

Looks like the regression was caused by XWIKI-11285.