Put the code in the page load of masterpage
protected void Page_Load(object sender, EventArgs e)
{
System.Globalization.CultureInfo ci = new System.Globalization.CultureInfo(Session["Language"].ToString());
System.Threading.Thread.CurrentThread.CurrentUICulture = ci;
}
Here Session["Language"] contains the language code. and it is stored to the session variable
from login page
Thanks