ASP.NET keeping the selected language in a multilingual website C#?

Question

This is my code, the main problem is when I select the language I want to, it changes to the language, but if I click to another web page it turns to the original one. Example: I select english, then it changes all the page to english but if I click to another link in the page, the language changes to the default language.

here is the code. Hope someone can help me.

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

    namespace Jaltepec
    {
        public partial class MainMaster : System.Web.UI.MasterPage
        {
            protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                cargaComboLenguajes();
            }
        }


        private void cargaComboLenguajes()
        {
            String lenguage = Request.QueryString["lenguaje"] == null ? "" : Request.QueryString["lenguaje"];
            if (lenguage == "en")
            {
                cmbIdioma.SelectedValue = "en-us";
            }
            else
            {
                cmbIdioma.SelectedValue = "es-mx";
            }            

        }
        protected void cmbIdioma_SelectedIndexChanged(object sender, EventArgs e)
        {
            string language = cmbIdioma.SelectedValue;

            if (!string.IsNullOrEmpty(language))
            {
                if (language.EndsWith("en-us"))
                {
                    Response.Redirect("Default.aspx?lenguaje=en");
                }
                else
                {
                    Response.Redirect("Default.aspx?lenguaje=es");
                }
            }
        }

    }
}

Answer 1

    private void cargaComboLenguajes()
    {
        if (Session["cmbIdioma"].ToString() != "")
        {
            cmbIdioma.SelectedValue = Session["cmbIdioma"].ToString();

        }

        else
        {
            cmbIdioma.SelectedValue = "en-us";
            Session["cmbIdioma"] = "en-us";
        }

    }
    protected void cmbIdioma_SelectedIndexChanged(object sender, EventArgs e)
    {
        string language = cmbIdioma.SelectedValue;

        if (!string.IsNullOrEmpty(language))
        {

            Session["cmbIdioma"] = cmbIdioma.SelectedValue.ToString();
            Response.Redirect("Default.aspx?lenguaje=" + Session["cmbIdioma"].ToString());

        }
    }