Designer
The designer view allows you to change the language of the Standalone Designer.
A drop down lists all languages available in the report server languages directory - _TelerikReportServerInstallationDirectory_\Telerik.ReportServer.Web\Report Designer\Languages\_CultureName_
. By default, for the German language this would be C:\Program Files (c86)\Progress\Telerik Report Server\Telerik.ReportServer.Web\Report Designer\Languages\de
.
For more information see Localization article.