Thank you for your reply.
"Telerik controls installation wizard automatically copies the built-in resources to the App_GlobalResources in your local installation."
The App_GlobalResources folder didn't have the RadGantt.Main.resx file.
I have added the required.resx files in the App_GlobalResources folder:
With the following fields: HeaderDay/HeaderWeek/HeaderMonth/HeaderYear
And set the Gantt culture as in the example:
- RadGantt1.Culture = CultureInfo.CreateSpecificCulture(Session["Culture"].ToString());
- Session["Culture"] is set to "de-DE"