Index: src/wp-includes/comment-template.php
===================================================================
--- src/wp-includes/comment-template.php	(revision 35536)
+++ src/wp-includes/comment-template.php	(working copy)
@@ -871,11 +871,25 @@
 	if ( $number > 1 ) {
 		if ( false === $more ) {
 			/* translators: %s: number of comments */
-			$output = sprintf( _n( '%s Comment', '%s Comments', $number ), number_format_i18n( $number ) );
-		} else {
-			// % Comments
-			$output = str_replace( '%', number_format_i18n( $number ), $more );
+			$more = sprintf( _n( '%s Comment', '%s Comments', $number ), number_format_i18n( $number ) );
+		} elseif ( 'on' === _x( 'off', 'Comment number declension: on or off' ) ) {
+			$text = preg_replace( '#<span class="screen-reader-text">.+?</span>#', '', $more );
+			$text = preg_replace( '/&.+?;/', '', $text ); // Kill entities
+			$text = trim( strip_tags( $text ), '% ' );
+
+			// Replace '% Comments' with a proper plural form
+			if ( $text && false !== strpos( $more, '%' ) ) {
+				/* translators: %s: number of comments */
+				$new_text = _n( '%s Comment', '%s Comments', $number );
+				$new_text = trim( sprintf( $new_text, '' ) );
+
+				$more = str_replace( $text, $new_text, $more );
+				if ( false === strpos( $more, '%' ) ) {
+					$more = '% ' . $more;
+				}
+			}
 		}
+		$output = str_replace( '%', number_format_i18n( $number ), $more );
 	} elseif ( $number == 0 ) {
 		$output = ( false === $zero ) ? __( 'No Comments' ) : $zero;
 	} else { // must be one
Index: tests/phpunit/tests/comment/template.php
===================================================================
--- tests/phpunit/tests/comment/template.php	(revision 35533)
+++ tests/phpunit/tests/comment/template.php	(working copy)
@@ -30,4 +30,112 @@
 		$this->assertEquals( 12, get_comments_number() );
 	}
 
-}
\ No newline at end of file
+	/**
+	 * @ticket 13651
+	 */
+	function test_get_comments_number_text_declension_with_default_args() {
+		$post_id = $this->factory->post->create();
+		$permalink = get_permalink( $post_id );
+		$this->go_to( $permalink );
+
+		$this->assertEquals( __( 'No Comments' ), get_comments_number_text() );
+
+		$this->factory->comment->create_post_comments( $post_id, 1 );
+		$this->go_to( $permalink );
+
+		$this->assertEquals( __( '1 Comment' ), get_comments_number_text() );
+
+		$this->factory->comment->create_post_comments( $post_id, 1 );
+		$this->go_to( $permalink );
+
+		$this->assertEquals( sprintf( _n( '%s Comment', '%s Comments', 2 ), '2' ), get_comments_number_text() );
+
+	}
+
+	/**
+	 * @ticket 13651
+	 * @dataProvider data_get_comments_number_text_declension
+	 */
+	function test_get_comments_number_text_declension_with_custom_args( $number, $input, $output ) {
+		$post_id = $this->factory->post->create();
+		$permalink = get_permalink( $post_id );
+
+		$this->factory->comment->create_post_comments( $post_id, $number );
+		$this->go_to( $permalink );
+
+		add_filter( 'gettext_with_context', array( $this, '_enable_comment_number_declension' ), 10, 4 );
+
+		$this->assertEquals( $output, get_comments_number_text( false, false, $input ) );
+
+		remove_filter( 'gettext_with_context', array( $this,  '_enable_comment_number_declension' ), 10, 4 );
+	}
+
+	function _enable_comment_number_declension( $translation, $text, $context, $domain ) {
+		if ( 'Comment number declension: on or off' === $context ) {
+			$translation = 'on';
+		}
+
+		return $translation;
+	}
+
+	function data_get_comments_number_text_declension() {
+		return array(
+			array(
+				2,
+				'Comments (%)',
+				sprintf( _n( '%s Comment', '%s Comments', 2 ), '2' ),
+			),
+			array(
+				2,
+				'2 Comments',
+				'2 Comments',
+			),
+			array(
+				2,
+				'2 Comments<span class="screen-reader-text"> on Hello world!</span>',
+				'2 Comments<span class="screen-reader-text"> on Hello world!</span>',
+			),
+			array(
+				2,
+				__( '% Comments', 'twentyten' ),
+				sprintf( _n( '%s Comment', '%s Comments', 2 ), '2' ),
+			),
+			array(
+				2,
+				_x( '%', 'comments number', 'twentyeleven' ),
+				'2',
+			),
+			array(
+				2,
+				__( '<b>%</b> Replies', 'twentyeleven' ),
+				sprintf( _n( '%s Comment', '%s Comments', 2 ), '<b>2</b>' ),
+			),
+			array(
+				2,
+				__( '% <span class="reply">comments &rarr;</span>', 'twentyeleven' ),
+				sprintf( '2 <span class="reply">%s &rarr;</span>', trim( sprintf( _n( '%s Comment', '%s Comments', 2 ), '' ) ) ),
+			),
+			array(
+				2,
+				__( '% Replies', 'twentytwelve' ),
+				sprintf( _n( '%s Comment', '%s Comments', 2 ), '2' ),
+			),
+			array(
+				2,
+				__( 'View all % comments', 'twentythirteen' ),
+				sprintf( _n( '%s Comment', '%s Comments', 2 ), '2' ),
+			),
+			array(
+				2,
+				__( '% Comments', 'twentyfourteen' ),
+				sprintf( _n( '%s Comment', '%s Comments', 2 ), '2' ),
+			),
+			array(
+				2,
+				__( '% Comments', 'twentyfifteen' ),
+				sprintf( _n( '%s Comment', '%s Comments', 2 ), '2' ),
+			),
+		);
+	}
+
+}