The Zemanta’s Related Posts WordPress Plug-in works very well with qTranslate. But it does not with qTranslate-Plus. Since the original qTranslate is deprecated many blogger have to switch to qTranslate-Plus. Plug-ins which works well with the original plug-in does not with the successor. The reason is the prefix of qTranslate-Plus functions in comparison to the original qTranslate. I described this in my post qTranslate-Plus Migration.
You just have to find the appropriate files which contain the qTranslate function calls. But a lot of plug-ins contain a lot of files. So it is hard to find the files in question. Zemanta’s Related Posts plug-in is such an example.