Reported for: WPML Multilingual CMS 3.2.7
Resolved in: 3.3
Overview of the issue
When using WordPress SEO by Yoast, separate sitemaps for each language should be created only when using a different domain per language.
Currently, every language has its own sitemap no matter which language URL format is selected (i.e. different languages in directories, a different domain per language, language name added as a parameter).
E.g. you have domain.com/sitemap.xml for the default language, domain.com/secondary-language-code/sitemap.xml for the secondary language, and the main language sitemap contains posts only in the default language.
In order for it to be correct, there should only be a domain.com/sitemap.xml, which would contain posts from all the active languages on the website.
None at this time.