Multilang comments hightlight post

Let me sum up your need :
You want to display all the comments for one post whatever their language (ie merge the comments made for the same post in different languages).

This can be achieved in 3 steps :
1. Remove the filter on comments that loads only those in the current language
2. Modify the function that built the list of comments (comments_array)
3. Modify the function that count the comments (get_comments_number)

Theses 3 steps can be programmed in your functions.php
You’ll find here after the total code to be included in your functions.php
It is the consolidation of the code written at http://wordpress.org/support/topic/multilang-comment (that use modified fucntions of the WPML comment merging plugin.
Note that the functions that modify comments_array and get_comments_number are using WPML functions (icl_…) instead of Polylang ones (pll_…) as polylang contains a “compatibility” module (polylang/include/wpml-compat.php) that allow us to use WPML functions.

Here is the code to put in functions.php :

//Remove the filter
function polylang_remove_comments_filter() {
    global $wp_filter;
    global $polylang;
    remove_filter('comments_clauses', array(&$polylang, 'comments_clauses'));
}

function sort_merged_comments($a, $b) {
	return $a->comment_ID - $b->comment_ID;
}

//Get the comments from the same posts in different languages
function merge_comments($comments, $post_ID) {
	global $sitepress;
	remove_filter( 'comments_clauses', array( $sitepress, 'comments_clauses' ) );
	// get all the languages for which this post exists
	$languages = icl_get_languages('skip_missing=1');
	$post = get_post( $post_ID );
	$type = $post->post_type;
	foreach($languages as $code => $l) {
		// in $comments are already the comments from the current language
		if(!$l['active']) {
			$otherID = icl_object_id($post_ID, $type, false, $l['language_code']);
			$othercomments = get_comments( array('post_id' => $otherID, 'status' => 'approve', 'order' => 'ASC') );
			$comments = array_merge($comments, $othercomments);
		}
	}
	if ($languages) {
		// if we merged some comments in we need to reestablish an order
		usort($comments, 'sort_merged_comments');
	}
	//
	add_filter( 'comments_clauses', array( $sitepress, 'comments_clauses' ) );

	return $comments;
}

//Add comments for the same posts in different languages
function merge_comment_count($count, $post_ID) {
	// get all the languages for which this post exists
	$languages = icl_get_languages('skip_missing=1');
	$languages = pll_the_languages(array('raw'=>1 ));
	$post = get_post( $post_ID );
	$type = $post->post_type;

	foreach($languages as $l) {
		// in $count is already the count from the current language
		if(!$l['active']) {
			$otherID = icl_object_id($post_ID, $type, false, $l['language_code']);
			if($otherID) {
				// cannot use call_user_func due to php regressions
				if ($type == 'page') {
					$otherpost = get_page($otherID);
				} else {
					$otherpost = get_post($otherID);
				}
				if ($otherpost) {
					// increment comment count using translation post comment count.
					$count = $count + $otherpost->comment_count;
				}
			}
		}
	}
	return $count;
}

//Trigger the functions
add_action('wp','polylang_remove_comments_filter');
add_filter('comments_array', 'merge_comments', 100, 2);
add_filter('get_comments_number', 'merge_comment_count', 100, 2);

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Tell us if it is working as you want.
If yes, do not forget to tag this thread as [Resolved]

It’s not working for me. I am using it for a page, not for posts. Does that make a difference?

I see the right number displayed of the merged comments (2324 comments) on both pages.
The comments themselves are not merged however:
One page shows 2323 comments.
The other page shows just one 1 test comment.