Here is a snippet how i create the bar:

class MainWindow : public QMainWindow
{
    Q_OBJECT
//...
private:
//...
    QMenu *mLanguageMenu;

};

void MainWindow::createMenus()
{
    qDebug() << menuBar()->actions().size();  // here its zero
    
    createLanguageMenu();

    menuBar()->addMenu(mLanguageMenu);

    qDebug() << menuBar()->actions().size();  // here size is 1 so it was added
}

void MainWindow::createLanguageMenu()
{
    mLanguageMenu = new QMenu{ this };

    mLanguageActionGroup = new QActionGroup{ this };

    connect(mLanguageActionGroup, &QActionGroup::triggered,
            this, &MainWindow::switchLanguage);

    QDir qmDir{":/translations"};
    auto fileNames = qmDir.entryList(QStringList("quiz_*.qm"));

    for (auto i=0; i < fileNames.size(); ++i) {
        auto locale = fileNames[i];
        locale.remove(0, locale.indexOf('_') + 1);
        locale.chop(3);

        QTranslator translator;
        translator.load(fileNames[i], qmDir.absolutePath());
        auto language = translator.translate("MainWindow", "English");

        auto action = new QAction{tr("&%1 %2").arg(i + 1).arg(language), this};

        action->setCheckable(true);
        action->setData(locale);

        mLanguageMenu->addAction(action);
        mLanguageActionGroup->addAction(action);

        if (language == "English") {
            action->setChecked(true);
        }
    }
}

At first i thougth this was maybe a bug in KDE Neon but then i also compiled the code with Windows 10 but its the same result. Any Idea what i do wrong? As you can see in the qDebug() statements it looks like the action gets added but does not show.