lang (and xml:lang) attribute in generated HTML pages. If {UseLocale} is enabled, it is calculated from the configure Internationalization tab → Locale sub-tab setting of {Site}{Locale}. Otherwise it defaults to en (English). lang attribute is defined like this: <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="%LANG%" lang="%LANG%">
LANG with LANGUAGE
%LANG% expands to en