What Is the Problem ? :) the language is changed once and never change again :)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Configuration;
using System.Threading;
using System.Globalization;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            InitializeCulture();
        }
    }
    protected override void InitializeCulture()
    {
        if (Session["Drop"] == "ar-SA")
        {
            Thread.CurrentThread.CurrentCulture = CultureInfo.CreateSpecificCulture("ar-SA");
            Thread.CurrentThread.CurrentUICulture = new CultureInfo("ar-SA");
        }
        else if (Session["Drop"] == "en-US")
        {
            Thread.CurrentThread.CurrentCulture = CultureInfo.CreateSpecificCulture("en-US");
            Thread.CurrentThread.CurrentUICulture = new CultureInfo("en-US");
        }
        else
        {
            Thread.CurrentThread.CurrentCulture = CultureInfo.CreateSpecificCulture("en-US");
            Thread.CurrentThread.CurrentUICulture = new CultureInfo("en-US");       
        }
        base.InitializeCulture();
    }

    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        Session["Drop"] = DropDownList1.SelectedValue;
        Response.Redirect("Default.aspx");
    }
}