MyClass::MyClass() {
    createWidgets();
    retranslateInterface();
}

MyClass::createWidgets() {
    whatever = new MyCustomWidget(this);
}

void MyClass::retranslateInterface() {
    whatever->setText(tr("blablabla"));
}

void MyClass::changeEvent(QEvent *event) {
    if (event->type() == QEvent::LanguageChange) {
        retranslateInterface();
    }
    SuperClass::changeEvent(event);
}