The upcoming version will support WPML, currently a workaround is needed.
I'm not sure if this is going to work, but it's worth a try.
1. Open up the wp-content/plugins/related-posts-pro/includes/related_content.class.php file
2. Make a backup copy of it, in case something goes wrong
3. Replace the file contents with this: http://pastebin.com/raw.php?i=Te1cyUiq
4. Go to your site back-end and open up the related posts pro instance settings and save them without changing anything – so it will clear the cache
Hopefully it will display posts from only the matching language.Best,
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