To reproduce: 

Create a localization provider following the article - http://wwwsit.telerik.com/help/winforms/pivotgrid-localization-localization.html. When you start the app you will see that the PivotAggregateP0ofP1 is not used.

Workaround:

Use the formatting events to format the needed values - http://wwwsit.telerik.com/help/winforms/pivotgrid-formatting-appearance.html