qmlpreview: fix init locale issue

The init language was never found, because the
findValidI18nDirectoryAsUrl() uses the m_lastLoadedUrl
to find the translation file path.

Change-Id: I6e9b62f3d846795d68ddef5e3a4caf3e3d953c7c
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

src/plugins/qmlpreview/qmlpreviewconnectionmanager.h

@@ -78,6 +78,7 @@ private:
     QmlPreviewFileLoader m_fileLoader = nullptr;
     QmlPreviewFileClassifier m_fileClassifier = nullptr;
     QmlPreviewFpsHandler m_fpsHandler = nullptr;
+    QString m_initLocale;
 };
 
 } // namespace Internal