Summary:
the only remaining use was klocale:

- languageCodeToName now goes through qlocale. it's worse data than before,
  with variants not being properly mapped into native names.
  alas, what isn't worse when relying on posix locales :|
- readOptions' default setting of a langauge now iters the configured
  languages in klocalizedstring (which in turn comes from the LANGUAGE
  env var + some extra fallback sugar) to find a language we know. if none
  was found we continue to fall back to en.soundtheme

Test Plan: builds + loads correct default language based on what is configured in systemsettings + language list looks somewhat sane

Reviewers: aacid, #kde_games

Reviewed By: aacid

Subscribers: kde-games-devel

Differential Revision: https://phabricator.kde.org/D17560