Widget i18n Testing
Also from that Twitter thread is:
There are still issues though ("For Plasma’s specific behaviour changes, you can read the QML source code for each control" ? Seriously ?), and testing the translations doesn't work (should the target locale be installed ?)
Which is correct, the locale must be installed to test the translations. I don't seem to mention that:
https://develop.kde.org/docs/plasma/widget/translations-i18n/#testing-our-translations