Currently when a header value is empty the displayed text is "(blank)" regardless of the application's culture. Localize this value or allow the developer to set this value more easily.

If you want to manually localize the "(blank)" text you can define a RowHeaderTemplate and ColumnHeaderTemplate (or template selector) and replace the default text with its translation. You can do that via IValueConverter.
(http://docs.telerik.com/devtools/wpf/controls/radpivotgrid/styles-and-templates/templating-cells#using-custom-cell-and-header-templates)