The aggregates functions highlighted in the attached screenshots have common names that are used in the industry and as such they are not localized. This feature request is so that their names can be localized through global resx files along with other strings in the RadPivotGrid control.

In the meantime, you can use the ItemDataBound event to change cell texts based on your requirements. You can find similar examples here: https://demos.telerik.com/aspnet-ajax/pivotgrid/examples/appearance/conditionalformatting/defaultcs.aspx
For example, to change the Sum aggregate name in the column header, you can do this:
            else if (e.Cell is PivotGridColumnHeaderCell)
            {
                PivotGridColumnHeaderCell cell = e.Cell as PivotGridColumnHeaderCell;
                cell.Text = cell.Text.Replace("Sum ", "custom ");