Update 03.08.2017 : I noticed that. Some themes and jetpack version are not compatible with the solution I outlined below. That’s why I found the solution in this plug-in. “WordPress Related Post“. This is small, fast, handy and free.
This is original solution : http://miklasnjor.com/2015/jetpack-related-posts-qtranslate-x-fix/
I want to explain it to you in more detail with screenshots.
Open file manager; ftp program or cpanel. The path is below;
And find that file “jetpack-related-posts.php”
And replace this line:
return wp_strip_all_tags( $post_title );