xml:lang is a universal attribute allowed in all XML dialects to mark up the natural human language that an element contains. It's almost identical in usage to HTML's lang, but in conforming XML 1.0 documents, it does not allow the use of a null attribute value (xml:lang="") to indicate an unknown language. Instead, use xml:lang="und".
SVG also has a native lang attribute, but it's only available on certain elements. xml:lang can be used anywhere.