For the time being you can manually set the localization string for the "Add Custom Color" and "Current Color" buttons: 
		<telerik:RadEditor ID="RadEditor1" runat="server" RenderMode="Mobile" Language="de-DE" LocalizationPath="~/App_GlobalResources"></telerik:RadEditor>
		<script>
			var initialize = Telerik.Web.UI.Editor.MobileColorPicker.prototype.initialize;
			var addCustomColorTextLocalized = "Add Custom Color Localized";
			var currentColorTextLocalized = "Current Color Localized";

			Telerik.Web.UI.Editor.MobileColorPicker.prototype.initialize = function () {
				var colorPicker = this;
				initialize.call(colorPicker);
				
				colorPicker._addCustomColorText = addCustomColorTextLocalized;
				colorPicker._currentColorText = currentColorTextLocalized;
			}
		</script>