I solved it. You must provide a Resource "Syncfusion.ReportDesigner.Wpf.de.resx" and you must insert
"System.Threading.Thread.CurrentThread.CurrentUICulture = new System.Globalization.CultureInfo("de");" before InitializeComponents() in the constructor of the mainwindow. In the resourcefile you can translate all items. I found this solution in the examples. Start Syncfusion control panel and click on "wpf" and then "explore".
In the following explorerwindowsearch for "samples\ReportDesigner.Wpf\Localization". There you can find the resourcefile too. Dear reader, excuse my bad english