How can I change the lang in this tags that are generated by default i want to change whats mark in bold

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
and add it in this tag <html xmlns="http://www.w3.org/1999/xhtml"> for <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en>

I added already this tag <meta http-equiv="Content-Language" content="en"> but it wont change the lang in the ones above
and i have the document as utf-8

Thank You