Microsoft Excel mangles Diacritics in .csv files?

/**
   * Export an array as downladable Excel CSV
   * @param array   $header
   * @param array   $data
   * @param string  $filename
   */
  function toCSV($header, $data, $filename) {
    $sep  = "t";
    $eol  = "n";
    $csv  =  count($header) ? '"'. implode('"'.$sep.'"', $header).'"'.$eol : '';
    foreach($data as $line) {
      $csv .= '"'. implode('"'.$sep.'"', $line).'"'.$eol;
    }

    header('Content-Description: File Transfer');
    header('Content-Type: application/vnd.ms-excel');
    header('Content-Disposition: attachment; filename='.$filename.'.csv');
    header('Content-Transfer-Encoding: binary');
    header('Expires: 0');
    header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
    header('Pragma: public');
    header('Content-Length: '. strlen($csv));
    echo chr(255) . chr(254) . mb_convert_encoding($csv, 'UTF-16LE', 'UTF-8');
    exit;
  }

>>> open('oldmac.csv', 'rb').read()
'xefxbbxbfOld MacDonald had a farm,xc3x88xc3x8cxc3x89xc3x8dxc3x98rn'
>>> ^Z

<%@LANGUAGE="JAVASCRIPT" CODEPAGE="65001"%>
<%
Response.Clear();
Response.ContentType = "text/csv";
Response.Charset = "utf-8";
Response.AddHeader("Content-Disposition", "attachment; filename=excelTest.csv");
Response.Write("uFEFF");
// csv text here
%>

<html>
<meta http-equiv="Content-Type" content="text/html" charset="utf-8" />
<table>
<tr>
  <th>id</th>
  <th>name</th>
</tr>
<tr>
 <td>4</td>
 <td>Hélène</td>
</tr>
</table>
</html>

def handlePersoonListExport(request):
    # Retrieve a query_set
    ...

    template = loader.get_template("export.csv")
    context = Context({
        'data': query_set,
    })

    response = HttpResponse()
    response['Content-Disposition'] = 'attachment; filename=export.csv'
    response['Content-Type'] = 'text/csv; charset=utf-8'
    response.write("xEFxBBxBF")
    response.write(template.render(context))

    return response

echo "xEFxBBxBF";

<%
      require 'fastercsv'
      fcsv_options = {
        :row_sep => "n",
        :col_sep => params[:delimiter],
        :force_quotes => @export_config.force_quotes,
        :headers => @export_columns.collect { |column| format_export_column_header_name(column) }
      }

      data = FasterCSV.generate(fcsv_options) do |csv|
        csv << fcsv_options[:headers] unless params[:skip_header] == 'true'
        @records.each do |record|
          csv << @export_columns.collect { |column|
            # Converte ogni campo in UTF-16 scartando il BOM, richiesto per Excel (> 2003 ?)
            Iconv.conv('UTF-16', 'UTF-8', get_export_column_value(record, column))[2..-1]
          }
        end
      end
    -%><%= data -%>

Iconv.conv('UTF-16', 'UTF-8', get_export_column_value(record, column))[2..-1]