I found it myself
The toolkit scriptmanager needs to be explicitly told that is has to use localisation:
<ajaxControlToolkit:ToolkitScriptManager ID="ToolkitScriptManager1" EnableScriptGlobalization="true" EnableScriptLocalization="true" runat="server" CombineScripts="true"></ajaxControlToolkit:ToolkitScriptManager>
After i changed this, the calendar was showing the date descriptions in Dutch