Sorry but the plugin you have patched seems to have some problems:
if a post is published only in one language out of the default, the sitemap will contain two URLs, one pointing to the correct post version, one pointing to the wrong default language:
example, at mysite.com, default language EN (english) , and additional language DE (german), there's a post with permalink "a-german-post"; in sitemap.xml you have:
http://mysite.com/de/a-german-post (CORRECT)
but also
http://mysite.com/a-german-post (WRONG)
Check it out
