How to use retranslateUi() recursively on all ui in QMainWindow?

In my Qt5 application I am trying to switch the language at run-time. So far here my simple function:

QTranslator _translator;

void MainWindow::switchLanguage(QString lang)
{
    if (!_translator.isEmpty()) qApp->removeTranslator(&_translator);
    if (lang == "it")
    {
        _translator.load("Language_it_IT.qm", qApp->applicationDirPath());
        qApp->installTranslator(&_translator);
    }
    ui->retranslateUi(this);
}

It works fine, but for the MainWindow only. Inside I have some form classes (with their UIs) that I load into a QStackedWidget:

FormMonitor _formMonitor;    // they all inherit from QWidget
FormSettings _formSettings;  // and have their .ui file
FormProgram _formProgram;

MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent), ui(new Ui::MainWindow)
{
    // ...
    ui->stackedWidget->addWidget(&_formMonitor);
    ui->stackedWidget->addWidget(&_formSettings);
    ui->stackedWidget->addWidget(&_formProgram);


    // ...
}

void MainWindow::loadView(int index)
{
    ui->stackedWidget->setCurrentIndex(index);
}

These forms are not translated.

How can I translate all the "nested" forms inside my MainWindow?

Reimplement the changeEvent of your form classes like this:

void Form::changeEvent(QEvent *e)
{
    if (e->type() == QEvent::LanguageChange)
        ui->retranslateUi(this);

    QWidget::changeEvent(e);
}