Unicode Technical Standard #35 introduces the Locale Data Markup Language (LDML), an XML format used for exchanging structured locale data. Developed by Mark Davis in 2004, it has been reviewed and approved by the Unicode Locale Data Technical Committee and can be referenced in other specifications for user locale information. The document contains details about locale IDs, inheritance, XML format, and several other technical aspects related to LDML.
UTR #35: Locale Data Markup Language
http://unicode.org/reports/tr35/tr35-4.html#DateFormatPatterns