App: Retranslate page header titles

This fixes a bug that is only practically effects the title of the
language page not updating the header when changing the language.

launcher/ui/widgets/PageContainer.h

@@ -66,8 +66,11 @@ public:
         m_container = container;
     };
 
+    void changeEvent(QEvent*) override;
+
 private:
     void createUI();
+    void retranslate();
 
 public slots:
     void help();