How do I solve question mark in arabic character

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ASP.NET

convert

hello, the following code is to convert from English to Arabic but somehow the Arabic appear as a question mark, but when converting from arbic to English it's work I don't if some one can help me, I will be appreciate

this is for design code

<form id="form1" runat="server">
    <div>
        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
        <asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" />
        <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
    </div>
    </form>

and this is for code behind

What I have tried:

<pre>using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class test : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
     protected void Button1_Click(object sender, EventArgs e)
        {
            TranslateText(TextBox1.Text, "en|ar");//Hindi to English
        }

        public string TranslateText(string input, string languagePair)
        {

            string url = String.Format("http://www.google.com/translate_t?hl=ar&en=UTF8&text={0}&langpair={1}", input, languagePair);

            System.Net.WebClient webClient = new System.Net.WebClient();

            webClient.Encoding = System.Text.Encoding.UTF8;




            string result = webClient.DownloadString(url);
            int index1 = Convert.ToInt32(result.IndexOf("onmouseout=" + '"' + "this." + "style." + "backgroundColor" + "=" + "'" + "#fff" + "'" + '"' + ">"));//onmouseout="this.style.backgroundColor='#fff'">
            int index2 = Convert.ToInt32(result.IndexOf("id=" + '"' + "gt-edit" + '"'));
            int index3 = index2 - index1;
            result = result.Substring((index1 + 47), (index3 - 49));
            Label1.Text = result.ToString();

            return result;

        }

    }

Thank u...