Load QTranslator-based translations for our override languages

I cannot figure out if there is some KF5 API that is supposed to do this for us, so I implemented the code to do this properly.

Fixes: https://bugs.kde.org/show_bug.cgi?id=395338 Fixes: https://bugs.kde.org/show_bug.cgi?id=363737

Test Plan

• Build the package (don't try to test it without packaging), then check that the "configure Krita" and "switch language" dialogs have localized button text.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• n/a I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.