We can localize Javascript pop up texts using server side variables in client side. In page class we will declare a protected variable:

 public partial class _Default : BasePage
 {
     protected string jsPopUpText;
 
     protected void Page_Load(object sender, EventArgs e)
     {
         jsPopUpText= Resources.TestSiteResources.jsPopUp;
     }
  

Then we will use this value in the client side JS code as:

 <script language="javascript">
 alert('<%= jsPopUpText %>');
 </script>

This will show us the localized text value from the RESX file based on the current thread's locale. For more info on localization in ASP.NET, refer this article:

http://www.codeproject.com/KB/aspnet/localizationByVivekTakur.aspx