I have customize plugin for WordPress Google XML Sitemaps (author Arne Brachhold) in order to put in sitemap.xml all localized links to posts, my multilingual blog based on Polyglot plugin (author Martin Chlupac).
You can download plugin with source code. google-sitemap-generator_polyglot.zip
Licence: GNU GPL
Tested on wordpress 2.9
Special thanks to Qian Qin