WPML is not an auto-translate tool, but one where authors can manually enter multiple posts in multiple languages. They appear as separate posts/pages in the WordPress database.
It's a very well-designed plugin. Basically WPML adds filters so that functions like
wp_list_pages only return pages in the currently selected language, and
get_permalink returns the correct URL for the currently selected language.
The problem with Sitemap plugins is that because of these filters, they only generate for the default language, and not for any alternate language.
Adapting your plugin would mean getting the list of languages (there's a function for that), and looping through the languages using your normal code for each language. The only difficulty I could anticipate would be persuading WPML to give you the posts for one language when running as a user using another language. I think you'd probably have to access the tables directly instead of using functions like
wp_list_pages, but I'm not sure.
If you have specific questions, please ask.