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