Language output is not correct when exporting to excel

Question

Hello good day! I have a webform application and I exported its content to an excel format. The language saved in my SQL Server is Lao script, but when I exported to excel it becomes unreadable. Below is the gridview before exporting to excel.

 

Below is the exported data in excel:

 

below is my code when exporting to excel:

protected void btnExport_Click(object sender, EventArgs e)
        {
            Response.ClearContent();
            Response.AppendHeader("content-disposition", "attachment; filename=IssuedTaxInvoices.xls");
            Response.ContentType = "application/excel";

            StringWriter stringWriter = new StringWriter();
            Html32TextWriter htmlTextWriter = new Html32TextWriter(stringWriter);

            //Below code will format the color only to the affected area
            GridView1.HeaderRow.Style.Add("background-color", "#FFFFFF");

            foreach (TableCell tableCell in GridView1.HeaderRow.Cells)
            {
                tableCell.Style["background-color"] = "#000084";
            }

            foreach (GridViewRow gridViewRow in GridView1.Rows)
            {
                gridViewRow.BackColor = System.Drawing.Color.White;
                foreach (TableCell gridviewRowTableCell in gridViewRow.Cells)
                {
                    gridviewRowTableCell.Style["background-color"] = "#EEEEEE";
                }
            }

            GridView1.FooterRow.Style.Add("background-color", "#FFFFFF");

            foreach (TableCell tableCell in GridView1.FooterRow.Cells)
            {
                tableCell.Style["background-color"] = "#CCCCCC";
            }

            foreach (GridViewRow gridViewRow in GridView1.Rows)
            {
                gridViewRow.BackColor = System.Drawing.Color.White;
                foreach (TableCell gridviewRowTableCell in gridViewRow.Cells)
                {
                    gridviewRowTableCell.Style["background-color"] = "#EEEEEE";
                }
            }

            //formatting of color ends here

            GridView1.RenderControl(htmlTextWriter);
            Response.Write(stringWriter.ToString());
            Response.End();
        }

 thanks in advance for any help.