under the CMS folder.
CMS/CMSPages/PortalTemplate.aspx
Edit the line:
<html <%=XHtmlNameSpace%> <%=XmlNamespace%>>
to
<html <%=XHtmlNameSpace%> <%=XmlNamespace%> <%=OtherHeaderTags%>>
Then in the PortalTemplate.aspx.cs
file, edit the OnPreRender as below:
protected override void OnPreRender(EventArgs e)
{
base.OnPreRender(e);
// Init the header tags
tags.Text = HeaderTags;
OtherHeaderTags = string.Format("lang=\"{0}\" xml:lang=\"{0}\"", LocalizationContext.CurrentCulture.CultureCode.Split('-')[0]);
}