#include <QApplication>
#include <QDoubleSpinBox>

int main(int argc, char **argv)
{
    QApplication app(argc, argv);

    QDoubleSpinBox spinBox;
    spinBox.show();

    return QApplication::exec();
}

Run this application and change the decimal separator using the Control Panel. It is expected that the spin box text will change but it doesn't.

With Qt 4.8 it does change because the notification — WM_SETTINGCHANGE with null wParam and 'intl' lParam — is handled here; I don't see anything similar in Qt 5 code.