I'm assuming you're using Portal Engine. You can add this code to the master page template at the beginning of the layout.
<script runat="server">
protected void Page_Load(object sender, EventArgs e)
{
if (CurrentDocument != null)
{
CMS.UIControls.ContentPage page= this.Page as CMS.UIControls.ContentPage;
if (page != null)
{
string lang= CMS.Localization.LocalizationContext.CurrentCulture.CultureCode;
page.XmlNamespace += " lang=\"" + lang + "\"";
}
}
}
</script>
This is editing the master page portal engine and not the base Kentico master template.