To reproduce:

Open the examples and navigate to GridView -> RightToLeft and toggle on RightToLeft. You will see that the GridGroupHeaderRowElements will have its text LeftToRight.

Workaround:

Use ViewCellFormatting:

private void Grid_ViewCellFormatting15(object sender, CellFormattingEventArgs e)
{
    if (e.CellElement.RowElement is GridGroupHeaderRowElement && e.CellElement.RowElement.Children.Any() && e.CellElement.RowElement.Children.Last() is GridGroupContentCellElement)
    {
        (e.CellElement.RowElement.Children.Last() as GridGroupContentCellElement).TextAlignment = System.Drawing.ContentAlignment.MiddleRight;
    }
}