What I want to say, is that if there is an english translation, it should say: "This post is also available in English" regardless of the post's language (and the system language of the visitor), so if an english speaker visit the site, he can understand the message, (as it's for him, not for the spanish speakers visitors).
Maybe it is not very important with english, but if you find a post in ukrainian, you will not understand de ukrainian equivalent to "This post is also available in English", what makes it useless.
I understand the problem with more than 2 languages... maybe this is the situation where the visitor's system language detection could be usefull. Or a configurable prefered language selection.
I'll continue studying the behavior of your plugin, is very complex and interesting. Thanks.