An example usage of QML internationalization and a helper C++ class

example.qml

import QtQuick 2.6
import QtQuick.Window 2.2
import TranslationHelper 1.0

Window {
    visible: true
    property int turn: 1

    TranslationHelper {
        id: translationTest
    }

    MouseArea {
        anchors.fill: parent
        acceptedButtons: Qt.LeftButton | Qt.RightButton
        onClicked: {
            if (mouse.button == Qt.RightButton) {
                translationTest.loadSystemLanguage();
            }
            else {
                if (turn === 1) {
                    translationTest.selectLanguage("en_US");
                    turn = 2;
                }
                else if (turn === 2) {
                    translationTest.selectLanguage("fr_FR");
                    turn = 1;
                }
            }
        }
    }

    Text {
        // When LanguageChangedSignal is emitted, qsTr's are re-evaluated
        text: qsTr("Merhaba Dünya!") + translationTest.LanguageChangedSignal
        anchors.centerIn: parent
    }
}

TranslationHelper.cpp

#include "TranslationHelper.h"
#include <QGuiApplication>
#include <QTranslator>

TranslationHelper::TranslationHelper(QObject *parent)
    : QObject(parent)
    , m_Translator(new QTranslator(this))
    , m_SourceLang("tr_TR")
    , m_CurrentLang(m_SourceLang)
    , m_TranslationsDir("translations")
{
    loadSystemLanguage();
}

QString TranslationHelper::getLanguageChangedSignal()
{
    return "";
}

void TranslationHelper::setSourceLang(const QString &lang)
{
    m_SourceLang = lang;
}

QString TranslationHelper::getSourceLang() const
{
    return m_SourceLang;
}

void TranslationHelper::selectLanguage(const QString &language)
{
    if (m_CurrentLang == language) {
        return;
    }

    if (m_SourceLang == language) {
        removeTranslator();
        return;
    }

    if (m_Translator->load(language, m_TranslationsDir)) {
        m_CurrentLang = language;
        qApp->installTranslator(m_Translator);
        emit languageChanged();
    }
}

QString TranslationHelper::getCurrentLang() const
{
    return m_CurrentLang;
}

void TranslationHelper::loadSystemLanguage()
{
    selectLanguage(QLocale::system().name());
}

void TranslationHelper::removeTranslator()
{
    if (qApp->removeTranslator(m_Translator)) {
        emit languageChanged();
    }
}

QString TranslationHelper::getTranslationsDir() const
{
    return m_TranslationsDir;
}

void TranslationHelper::setTranslationsDir(const QString &dir)
{
    m_TranslationsDir = dir;
}

TranslationHelper.h

#ifndef TRANSLATIONTEST_H
#define TRANSLATIONTEST_H
#include <QObject>
class QTranslator;

class TranslationHelper : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString LanguageChangedSignal READ getLanguageChangedSignal NOTIFY languageChanged)

public:
    explicit TranslationHelper(QObject *parent = 0);
    QString getLanguageChangedSignal();

    /**
     * @brief Sets the source language. The source language does not have a *.qm file so If the language you requested to load equals to the source language
     * the translator is removed to return to source language
     * @param lang
     */
    Q_INVOKABLE void setSourceLang(const QString &lang);
    Q_INVOKABLE QString getSourceLang() const;

    Q_INVOKABLE void selectLanguage(const QString &language);
    Q_INVOKABLE QString getCurrentLang() const;
    Q_INVOKABLE void loadSystemLanguage();

    /**
     * @brief Removes the translator from the application, making it return to the source language
     */
    Q_INVOKABLE void removeTranslator();

    Q_INVOKABLE QString getTranslationsDir() const;
    Q_INVOKABLE void setTranslationsDir(const QString &dir);

signals:
    void languageChanged();

private:
    QTranslator *m_Translator;
    QString m_SourceLang,
            m_CurrentLang,
            m_TranslationsDir;
};

#endif // TRANSLATIONTEST_H