Got it!
Just included code into sem-theme-posts.php
Anyway, thanks a lot!
I´ve installed the Polyglot plugin and it works fine. But I could not call it from inside of my posts. Therefore, it won´t show up that little language flags if a post is called from within category listing. I use the Semiologic 2 col theme. In this example http://www.texxtus.net/wd_press/archives/71 it can be seen. (home is http://www.texxtus.net)
Question: where do I put the code to call the plugin? I put it into index.php, just before the the_posts() call.