1. Open up your Website in Visual Studio
2. Open up Global.asax and add these lines

   void Session_Start(object sender, EventArgs e)
   {
       /*
        * Set the default language of the website
        * In my case it is en-CA, but you could put anything
        */
       Session["MyCulture"] = "en-CA";
   }

3. Create two links in your MasterPage.master

   <asp:linkbutton
       onclick="changeCulture_Click"
       commandargument="fr-CA"
       text="Français"
       runat="server"
       id="btnSetFR"/>
   <asp:linkbutton
       onclick="changeCulture_Click"
       commandargument="en-CA"
       text="English"
       runat="server"
       id="btnSetEN"/>

4. Add the Click Event to your MasterPage.master.cs

       protected void changeCulture_Click(object sender,
          EventArgs e)
       {
           LinkButton senderLink = sender as
               LinkButton;
   
           Session["userCulture"] =
              senderLink.CommandArgument;
   
           Server.Transfer(Request.Path);
       }

5. Create a new C# Class in App_Code, name it BasePage.cs and copy this code in it.

       public class MyPage : Page
       {
           protected override void InitializeCulture()
           {
               /*
                * Get the culture from the Sessions
                */
               string currentCulture =
                   Convert.ToString(Session["userCulture"]);
   
               /*
                * Check if there's in a
                * culture in the Sessions
                * Otherwise, set the Default Culture
                */
               if (!string.IsNullOrEmpty(currentCulture))
                   currentCulture = currentCulture;
               else
                   currentCulture = "en-CA"; 
   
               /*
                * Apply the Culture to our thread.
                */
               Thread.CurrentThread.CurrentCulture =
                   CultureInfo.CreateSpecificCulture(
                       currentCulture
                   );
   
               Thread.CurrentThread.CurrentUICulture =
                   new CultureInfo(currentCulture);
   
               base.InitializeCulture();
           }
       }

6. Change the inheritance of all your web forms from Page to BasePage
7. Now all you need to do is create your Resources in GlobalResources and LocalResources Folders

As I said, I learned this from an article posted on CodeProject by Michael Ulmann. If you would like to read the original article or download the source code, follow this link