protected void Page_Load(object sender, EventArgs e)
{
   bool blnSomeCondition = false;
   // code to populate blnSomeCondition...

   if (blnSomeCondition)
   {
       string strTextFromResx = String.Empty;
       // read the text from the resource file into the strTextFromResx
variable
       ClientScript.RegisterStartupScript(GetType(), "resxMsg", alert('" +
strTextFromResx + "');", true);
   }
   else
   {
       // rest of Page_Load method...
   }
}

You can't do it, at least not directly.

If you emit the JavaScript, however, you can set it up based on the user's
language preference when the page is built.

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