The language of the intellectual content of the element for which this is an attribute, typically a whole book
Recommended best practice is to use values as defined in RFC 1766, typically two-letter language codes such as “FR” (French), “EN” (English), and “DE” (German), including hyphenated differentiators such as “EN-AU” (Australian English) and “EN-US” (US English). These values are NOT case sensitive, so “EN” equals “en”.
Processing and Conversion Note: Unlike nearly all other attributes in XML, the value of language is inherited. This means that all elements inside the one with the language attribute (its descendants) are assumed to be in the same language, unless they explicitly set their own xml:lang attribute.