add auto-detect system language in i18n example

It makes this example more complete since we move
linguist related examples into qttools

Task-number: QTBUG-28434
Change-Id: Ic6bbbd2702b5a0a304b5e8ce59da37ef95e4b42e
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

examples/widgets/doc/src/i18n.qdoc

@@ -32,7 +32,9 @@
 
     \brief The Internationalization (I18N) example demonstrates Qt's support for translated
     text. Developers can write the initial application text in one language, and
-    translations can be provided later without any modifications to the code.
+    translations can be provided later without any modifications to the code. It also
+    demonstrates how to detect the system language settings and show the UI in the appropriate
+    language.
 
     \image i18n-example.png
 */

examples/widgets/tools/i18n/languagechooser.cpp

@@ -49,7 +49,7 @@ extern void qt_mac_set_menubar_merge(bool merge);
 QT_END_NAMESPACE
 #endif
 
-LanguageChooser::LanguageChooser(QWidget *parent)
+LanguageChooser::LanguageChooser(const QString& defaultLang, QWidget *parent)
     : QDialog(parent, Qt::WindowStaysOnTopHint)
 {
     groupBox = new QGroupBox("Languages");
@@ -61,6 +61,8 @@ LanguageChooser::LanguageChooser(QWidget *parent)
         QCheckBox *checkBox = new QCheckBox(languageName(qmFiles[i]));
         qmFileForCheckBoxMap.insert(checkBox, qmFiles[i]);
         connect(checkBox, SIGNAL(toggled(bool)), this, SLOT(checkBoxToggled()));
+        if (languageMatch(defaultLang, qmFiles[i]))
+                checkBox->setCheckState(Qt::Checked);
         groupBoxLayout->addWidget(checkBox, i / 2, i % 2);
     }
     groupBox->setLayout(groupBoxLayout);
@@ -87,6 +89,14 @@ LanguageChooser::LanguageChooser(QWidget *parent)
     setWindowTitle("I18N");
 }
 
+bool LanguageChooser::languageMatch(const QString& lang, const QString& qmFile)
+{
+    //qmFile: i18n_xx.qm
+    const QString prefix = "i18n_";
+    const int langTokenLength = 2; /*FIXME: is checking two chars enough?*/
+    return qmFile.midRef(qmFile.indexOf(prefix) + prefix.length(), langTokenLength) == lang.leftRef(langTokenLength);
+}
+
 bool LanguageChooser::eventFilter(QObject *object, QEvent *event)
 {
     if (event->type() == QEvent::Close) {

examples/widgets/tools/i18n/languagechooser.h

@@ -58,7 +58,7 @@ class LanguageChooser : public QDialog
     Q_OBJECT
 
 public:
-    LanguageChooser(QWidget *parent = 0);
+    explicit LanguageChooser(const QString& defaultLang = QString(), QWidget *parent = 0);
 
 protected:
     bool eventFilter(QObject *object, QEvent *event);
@@ -73,6 +73,7 @@ private:
     QStringList findQmFiles();
     QString languageName(const QString &qmFile);
     QColor colorForLanguage(const QString &language);
+    static bool languageMatch(const QString& lang, const QString& qmFile);
 
     QGroupBox *groupBox;
     QDialogButtonBox *buttonBox;

examples/widgets/tools/i18n/main.cpp

@@ -39,7 +39,7 @@
 ****************************************************************************/
 
 #include <QApplication>
-
+#include <QLocale>
 #include "languagechooser.h"
 #include "mainwindow.h"
 
@@ -48,7 +48,7 @@ int main(int argc, char *argv[])
     Q_INIT_RESOURCE(i18n);
 
     QApplication app(argc, argv);
-    LanguageChooser chooser;
+    LanguageChooser chooser(QLocale::system().name());
     chooser.show();
     return app.exec();
 }