First, XML is a format to represent data. It has originally be designed for this data to be essentially text documents or messages, but nothing prevents you from representing any data you wish, including programs. There at least three big examples of programs expressed in XML that come to mind:
XSLT is a Turing-complete language whose only standard format is in XML, whose essential prupose is data transformations
ANT is a task manager, used mostly for compilation management, whose tasks are described in XML
GCC-XML is a format that GCC can use to output the result of its parsing of a source code
On a more esoteric note, one could cite o:XML, a full-fledged XML programming language.
So as any XML DTD or Schema semantically extends XML itself, one could technically argue that XML itself, through some of its extensions, is a programming language.
