QTranslator: why are some parts of app not translated?

Question

There is a working app and I am adding a new language for GUI. Everything works fine but some part of the app are simply not translated. QLinguist detects all of them, and I have added a new translations, but still no result.

Here is the fragment of the code that is not getting translation:

"imagecropwindow_p.h":

#include <QWidget>

class QLabel;
class QPushButton;
class QHBoxLayout;
class QVBoxLayout;
class QFrame;

class CropWindowComponents: public QWidget
{
public:
    CropWindowComponents(QWidget *parent = nullptr);

    QPushButton *changeBtn;
    QPushButton *cropBtn;
    QPushButton *continueBtn;
    QPushButton *cancelBtn;
};

class HorizontalWindow : public CropWindowComponents
{
    Q_OBJECT
public:
    HorizontalWindow(QWidget *parent = nullptr);
};

class VerticalWindow : public CropWindowComponents
{
    Q_OBJECT
public:
    VerticalWindow(QWidget *parent = nullptr);
};

"imagecropwindow_p.cpp":

#include "imagecropwindow_p.h"

#include <QLabel>
#include <QPushButton>
#include <QHBoxLayout>
#include <QVBoxLayout>

CropWindowComponents::CropWindowComponents(QWidget *parent) :
    QWidget(parent)
{
    changeBtn = new QPushButton(tr("Change"), this);
    cropBtn = new QPushButton(tr("Crop"), this);
    continueBtn = new QPushButton(tr("Continue"), this);
    cancelBtn = new QPushButton(tr("Cancel"), this);
}

HorizontalWindow::HorizontalWindow(QWidget *parent) :
    CropWindowComponents(parent)
{
    QHBoxLayout *btnsLyt = new QHBoxLayout;
    btnsLyt->setMargin(0);
    btnsLyt->addWidget(changeBtn);
    btnsLyt->addWidget(cropBtn);
    btnsLyt->addWidget(continueBtn);
    btnsLyt->addWidget(cancelBtn);
    btnsLyt->addStretch();

    setLayout(btnsLyt);
}

VerticalWindow::VerticalWindow(QWidget *parent) :
    CropWindowComponents(parent)
{
    QVBoxLayout *btnsLyt = new QVBoxLayout;
    btnsLyt->setMargin(0);
    btnsLyt->addWidget(changeBtn);
    btnsLyt->addWidget(cropBtn);
    btnsLyt->addWidget(continueBtn);
    btnsLyt->addWidget(cancelBtn);
    btnsLyt->addStretch();

    setLayout(btnsLyt);
}

"imagecropperwindow.h":

#include "imagecropwindow_p.h"

class ImageCropperWindow : public QWidget
{
    Q_OBJECT
public:
    explicit ImageCropperWindow(QWidget *parent = nullptr);

private slots:
    void changeWindowOrientation();

private:
    HorizontalWindow *horizWindow;
    VerticalWindow *verticalWindow;
};

"imagecropperwindow.cpp":

#include "imagecropperwindow.h"

#include <QDebug>
#include <QVBoxLayout>
#include <QPushButton>
#include <QLabel>

ImageCropperWindow::ImageCropperWindow(QWidget *parent) : QWidget(parent)
{
    horizWindow = new HorizontalWindow(this);
    verticalWindow = new VerticalWindow(this);

    connect(horizWindow->changeBtn, &QPushButton::clicked,
            this, &ImageCropperWindow::changeWindowOrientation);
    connect(verticalWindow->changeBtn, &QPushButton::clicked,
            this, &ImageCropperWindow::changeWindowOrientation);

    QVBoxLayout *layout = new QVBoxLayout(this);
    layout->setMargin(3);
    layout->addWidget(horizWindow);
    layout->addWidget(verticalWindow);
    verticalWindow->setVisible(false);

    setLayout(layout);
}

void ImageCropperWindow::changeWindowOrientation()
{
    if (horizWindow->isVisible()) {
        horizWindow->setVisible(false);
        verticalWindow->setVisible(true);
    }
    else {
        verticalWindow->setVisible(false);
        horizWindow->setVisible(true);
    }

    this->resize(this->minimumSizeHint());
}

And "main.cpp":

#include <QApplication>
#include <QDebug>
#include <QTranslator>

#include "imagecropperwindow.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QTranslator newLang;
    newLang.load(":/translations/newLanguage.qm");

    a.installTranslator(&newLang);

    ImageCropperWindow w;
    w.show();

    return a.exec();
}

You can find the whole project here.

PS: For an example case, I added stars (*) as a new translation, such as

nativeLanuage  - ***newLanguage***  
Change          - ***Change***  
Crop            - ***Crop***  
Continue        - ***Continue***  
Cancel          - ***Cancel*** 

Answer 1

QTranslator uses the MOC to do the translations so if you want that if you want your widget to be translated you should use the macro Q_OBJECT, in your case CropWindowComponents does not have it, so the solution is to add it:

imagecropwindow_p.h

class CropWindowComponents: public QWidget
{
    Q_OBJECT # <--- add this
public:
...

On the other hand do not add the .ts to the .qrc since the .ts only serves to convert it to the .qm binary. When you add a file to the .qrc it is compiled and added to the executable, increasing the size of the last one. Therefore adding the .ts increases the size of the executable unnecessarily.