Support locales with fallbacks
Summary: Jonathan uses en_GB.dict that diverges slightly from en.dict. They both should be taken into account properly.
Test Plan: Used Jon's env var
Reviewers: Plasma, jriddell
Reviewed By: Plasma, jriddell
Subscribers: jriddell, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D25823