Qt: Add translation support and language setting

src/duckstation-qt/mainwindow.cpp

@@ -463,6 +463,31 @@ void MainWindow::setupAdditionalUi()
     });
   }
   updateDebugMenuGPURenderer();
+
+  const QString current_language(
+    QString::fromStdString(m_host_interface->GetStringSettingValue("Main", "Language", "")));
+  for (const std::pair<QString, QString>& it : m_host_interface->getAvailableLanguageList())
+  {
+    QAction* action = m_ui.menuSettingsLanguage->addAction(it.first);
+    action->setCheckable(true);
+    action->setChecked(it.second == current_language);
+    action->setData(it.second);
+    connect(action, &QAction::triggered, [this, action]() {
+      const QString new_language = action->data().toString();
+      m_host_interface->SetStringSettingValue("Main", "Language", new_language.toUtf8().constData());
+      QMessageBox::information(this, tr("DuckStation"),
+                               tr("Language changed. Please restart the application to apply."));
+      for (QObject* obj : m_ui.menuSettingsLanguage->children())
+      {
+        QAction* other_action = qobject_cast<QAction*>(obj);
+        if (other_action)
+        {
+          QSignalBlocker blocker(other_action);
+          action->setChecked(other_action == action);
+        }
+      }
+    });
+  }
 }
 
 void MainWindow::updateEmulationActions(bool starting, bool running)

src/duckstation-qt/mainwindow.ui

@@ -90,6 +90,11 @@
       <string>Theme</string>
      </property>
     </widget>
+    <widget class="QMenu" name="menuSettingsLanguage">
+     <property name="title">
+      <string>Language</string>
+     </property>
+    </widget>
     <addaction name="actionFullscreen"/>
     <addaction name="separator"/>
     <addaction name="actionGeneralSettings"/>
@@ -106,6 +111,7 @@
     <addaction name="actionScanForNewGames"/>
     <addaction name="actionRescanAllGames"/>
     <addaction name="separator"/>
+    <addaction name="menuSettingsLanguage"/>
     <addaction name="menuSettingsTheme"/>
    </widget>
    <widget class="QMenu" name="menuHelp">

src/duckstation-qt/qthostinterface.cpp

@@ -23,7 +23,9 @@
 #include <QtCore/QDateTime>
 #include <QtCore/QDebug>
 #include <QtCore/QEventLoop>
+#include <QtCore/QFile>
 #include <QtCore/QTimer>
+#include <QtCore/QTranslator>
 #include <QtWidgets/QMenu>
 #include <QtWidgets/QMessageBox>
 #include <algorithm>
@@ -49,10 +51,19 @@ const char* QtHostInterface::GetFrontendName() const
   return "DuckStation Qt Frontend";
 }
 
+std::vector<std::pair<QString, QString>> QtHostInterface::getAvailableLanguageList()
+{
+  return {{QStringLiteral("English"), QStringLiteral("")}};
+}
+
 bool QtHostInterface::Initialize()
 {
   createThread();
-  return m_worker_thread->waitForInit();
+  if (!m_worker_thread->waitForInit())
+    return false;
+
+  installTranslator();
+  return true;
 }
 
 void QtHostInterface::Shutdown()
@@ -82,6 +93,36 @@ void QtHostInterface::shutdownOnThread()
   CommonHostInterface::Shutdown();
 }
 
+void QtHostInterface::installTranslator()
+{
+  m_translator = std::make_unique<QTranslator>();
+
+  std::string language = GetStringSettingValue("Main", "Language", "");
+  if (language.empty())
+    return;
+
+  const QString path =
+    QStringLiteral("%1/translations/duckstation-qt_%3.qm").arg(qApp->applicationDirPath()).arg(language.c_str());
+  if (!QFile::exists(path))
+  {
+    QMessageBox::warning(
+      nullptr, QStringLiteral("Translation Error"),
+      QStringLiteral("Failed to find translation file for language '%1':\n%2").arg(language.c_str()).arg(path));
+    return;
+  }
+
+  if (!m_translator->load(path))
+  {
+    QMessageBox::warning(
+      nullptr, QStringLiteral("Translation Error"),
+      QStringLiteral("Failed to load translation file for language '%1':\n%2").arg(language.c_str()).arg(path));
+    return;
+  }
+
+  Log_InfoPrintf("Loaded translation file for language '%s'", language.c_str());
+  qApp->installTranslator(m_translator.get());
+}
+
 void QtHostInterface::ReportError(const char* message)
 {
   HostInterface::ReportError(message);

src/duckstation-qt/qthostinterface.h

@@ -22,6 +22,7 @@ class QEventLoop;
 class QMenu;
 class QWidget;
 class QTimer;
+class QTranslator;
 
 class GameList;
 class INISettingsInterface;
@@ -95,6 +96,9 @@ public:
   /// Returns a path relative to the application directory (for system files).
   QString getProgramDirectoryRelativePath(const QString& arg) const;
 
+  /// Returns a list of supported languages and codes (suffixes for translation files).
+  static std::vector<std::pair<QString, QString>> getAvailableLanguageList();
+
 Q_SIGNALS:
   void errorReported(const QString& message);
   void messageReported(const QString& message);
@@ -213,6 +217,7 @@ private:
   void threadEntryPoint();
   bool initializeOnThread();
   void shutdownOnThread();
+  void installTranslator();
   void renderDisplay();
   void connectDisplaySignals(QtDisplayWidget* widget);
   void updateDisplayState();
@@ -222,6 +227,8 @@ private:
   std::unique_ptr<INISettingsInterface> m_settings_interface;
   std::recursive_mutex m_settings_mutex;
 
+  std::unique_ptr<QTranslator> m_translator;
+
   MainWindow* m_main_window = nullptr;
   QThread* m_original_thread = nullptr;
   Thread* m_worker_thread = nullptr;