There is a plugin for wordpress called XML Sitemap Feed what is very popular for creating detailed search engines friendly sitemap. It works with some translation plugins, but not with GTranslate Pro at the current moment.
Robots.txt with sitemap.txt with site.domain/lang/ works fine, but it will be great if you will add your plugin support to XML Sitemap Feed. I think it's easier for you then for them because you perfectly know how to get necessary data for their plugin to generate sitemaps for each enabled language.
Happy GTPro user.
Robots.txt with sitemap.txt with site.domain/lang/
This is the cat,
That killed the rat,
That ate the malt
That lay in the house that Jack built.