The problem seems to be that the QT 6 installation doesn't remove the old components before upgrading, so the old international resources are left after the upgrade! If your primary setting is not English, you end up using them with the new prefs panel (which is different from that of QT5, so this is not good!). I had to remove them manually, but I guess the best answer is to remove the QT 5 prefs pane (in /System -> Library -> PreferencePanes) before installing QT 6.
[Editor's note: I haven't installed QT6 on any system, English or non-English, so I can't specifically vouch for this hint. It seemed worth posting as a general heads-up, though, for those who are considering a QT6 Public Preview installation.]

