To reproduce:
- Populate the grid with some double values.
- Change the culture to Dutch(Belgian)
- Export the grid

Workaround:

void spreadExporter_CellFormatting(object sender, Telerik.WinControls.UI.Export.SpreadExport.CellFormattingEventArgs e)
{
    if (e.GridColumnIndex == 0 && e.GridCellInfo.Value != string.Empty)
    {
        CellValueFormat cvf = new CellValueFormat("0,00");
        e.CellSelection.SetFormat(cvf);
        e.CellSelection.SetValue(((double)e.GridCellInfo.Value));
    }
}