How to Make WordPress Slugs Look the Same in Both Language Versions with Free Polylang

This case is not super complicated, but I decided to write about it. When working with multilingual sites, we face a problem: we cannot write the same slugs on both language versions of an article. The free version of the Polylang plugin does not provide this feature. You can get around this drawback by using another plugin – Permalink Manager Lite. After activating the plugin, a field appears in the admin pane, it duplicates the slug from the native slug field in WordPress by default. However, it can be changed like the slug itself. The field looks like this:

An alternative way to work with slugs

On GitHub, there is a plugin by Ulrich Pogson called Polylang Slug. I tested it on the Blockera block theme and it works well. Below is the plugin code without version checking, it also works if added to the functions.php file:

function polylang_slug_unique_slug_in_language( $slug, $post_ID, $post_status, $post_type, $post_parent, $original_slug ){

	// Return slug if it was not changed.
	if ( $original_slug === $slug ) {
		return $slug;
	}

	global $wpdb;

	// Get language of a post
	$lang = pll_get_post_language( $post_ID );
	$options = get_option( 'polylang' );

	// return the slug if Polylang does not return post language or has incompatable redirect setting or is not translated post type.
	if ( empty( $lang ) || 0 === $options['force_lang'] || ! pll_is_translated_post_type( $post_type ) ) {
		return $slug;
	}

	// " INNER JOIN $wpdb->term_relationships AS pll_tr ON pll_tr.object_id = ID".
	$join_clause  = polylang_slug_model_post_join_clause();
	// " AND pll_tr.term_taxonomy_id IN (" . implode(',', $languages) . ")".
	$where_clause = polylang_slug_model_post_where_clause( $lang );

	// Polylang does not translate attachements - skip if it is one.
	// @TODO Recheck this with the Polylang settings
	if ( 'attachment' == $post_type ) {

		// Attachment slugs must be unique across all types.
		$check_sql = "SELECT post_name FROM $wpdb->posts $join_clause WHERE post_name = %s AND ID != %d $where_clause LIMIT 1";
		$post_name_check = $wpdb->get_var( $wpdb->prepare( $check_sql, $original_slug, $post_ID ) );

	} elseif ( is_post_type_hierarchical( $post_type ) ) {

		// Page slugs must be unique within their own trees. Pages are in a separate
		// namespace than posts so page slugs are allowed to overlap post slugs.
		$check_sql = "SELECT ID FROM $wpdb->posts $join_clause WHERE post_name = %s AND post_type IN ( %s, 'attachment' ) AND ID != %d AND post_parent = %d $where_clause LIMIT 1";
		$post_name_check = $wpdb->get_var( $wpdb->prepare( $check_sql, $original_slug, $post_type, $post_ID, $post_parent ) );

	} else {

		// Post slugs must be unique across all posts.
		$check_sql = "SELECT post_name FROM $wpdb->posts $join_clause WHERE post_name = %s AND post_type = %s AND ID != %d $where_clause LIMIT 1";
		$post_name_check = $wpdb->get_var( $wpdb->prepare( $check_sql, $original_slug, $post_type, $post_ID ) );

	}

	if ( ! $post_name_check ) {
		return $original_slug;
	}

	return $slug;
}
add_filter( 'wp_unique_post_slug', 'polylang_slug_unique_slug_in_language', 10, 6 );

/**
 * Modify the sql query to include checks for the current language.
 *
 * @since 0.1.0
 *
 * @global wpdb   $wpdb  WordPress database abstraction object.
 *
 * @param  string $query Database query.
 *
 * @return string        The modified query.
 */
function polylang_slug_filter_queries( $query ) {
	global $wpdb;

	// Query for posts page, pages, attachments and hierarchical CPT. This is the only possible place to make the change. The SQL query is set in get_page_by_path()
	$is_pages_sql = preg_match(
		"#SELECT ID, post_name, post_parent, post_type FROM {$wpdb->posts} .*#",
		polylang_slug_standardize_query( $query ),
		$matches
	);

	if ( ! $is_pages_sql ) {
		return $query;
	}

	// Check if should contine. Don't add $query polylang_slug_should_run() as $query is a SQL query.
	if ( ! polylang_slug_should_run() ) {
		return $query;
	}

	$lang = pll_current_language();
	// " INNER JOIN $wpdb->term_relationships AS pll_tr ON pll_tr.object_id = ID".
	$join_clause  = polylang_slug_model_post_join_clause();
	// " AND pll_tr.term_taxonomy_id IN (" . implode(',', $languages) . ")".
	$where_clause = polylang_slug_model_post_where_clause( $lang );

	$query = preg_match(
		"#(SELECT .* (?=FROM))(FROM .* (?=WHERE))(?:(WHERE .*(?=ORDER))|(WHERE .*$))(.*)#",
		polylang_slug_standardize_query( $query ),
		$matches
	);

	// Reindex array numerically $matches[3] and $matches[4] are not added together thus leaving a gap. With this $matches[5] moves up to $matches[4]
	$matches = array_values( $matches );

	// SELECT, FROM, INNER JOIN, WHERE, WHERE CLAUSE (additional), ORBER BY (if included)
	$sql_query = $matches[1] . $matches[2] . $join_clause . $matches[3] . $where_clause . $matches[4];

	/**
	 * Disable front end query modification.
	 *
	 * Allows disabling front end query modification if not needed.
	 *
	 * @since 0.2.0
	 *
	 * @param string $sql_query    Database query.
	 * @param array  $matches {
	 *     @type string $matches[1] SELECT SQL Query.
	 *     @type string $matches[2] FROM SQL Query.
	 *     @type string $matches[3] WHERE SQL Query.
	 *     @type string $matches[4] End of SQL Query (Possibly ORDER BY).
	 * }
	 * @param string $join_clause  INNER JOIN Polylang clause.
	 * @param string $where_clause Additional Polylang WHERE clause.
	 */
	return apply_filters( 'polylang_slug_sql_query', $sql_query, $matches, $join_clause, $where_clause );
}
add_filter( 'query', 'polylang_slug_filter_queries' );

/**
 * Extend the WHERE clause of the query.
 *
 * This allows the query to return only the posts of the current language
 *
 * @since 0.1.0
 *
 * @param  string   $where The WHERE clause of the query.
 * @param  WP_Query $query The WP_Query instance (passed by reference).
 *
 * @return string          The WHERE clause of the query.
 */
function polylang_slug_posts_where_filter( $where, $query ) {
	// Check if should contine.
	if ( ! polylang_slug_should_run( $query ) ) {
		return $where;
	}

	$lang = empty( $query->query['lang'] ) ? pll_current_language() : $query->query['lang'];

	// " AND pll_tr.term_taxonomy_id IN (" . implode(',', $languages) . ")"
	$where .= polylang_slug_model_post_where_clause( $lang  );

	return $where;
}
add_filter( 'posts_where', 'polylang_slug_posts_where_filter', 10, 2 );

/**
 * Extend the JOIN clause of the query.
 *
 * This allows the query to return only the posts of the current language
 *
 * @since 0.1.0
 *
 * @param  string   $join  The JOIN clause of the query.
 * @param  WP_Query $query The WP_Query instance (passed by reference).
 *
 * @return string          The JOIN clause of the query.
 */
function polylang_slug_posts_join_filter( $join, $query ) {

	// Check if should contine.
	if ( ! polylang_slug_should_run( $query ) ) {
		return $join;
	}

	// " INNER JOIN $wpdb->term_relationships AS pll_tr ON pll_tr.object_id = ID".
	$join .= polylang_slug_model_post_join_clause();

	return $join;
}
add_filter( 'posts_join', 'polylang_slug_posts_join_filter', 10, 2 );

/**
 * Check if the query needs to be adapted.
 *
 * @since 0.2.0
 *
 * @param  WP_Query $query The WP_Query instance (passed by reference).
 *
 * @return bool
 */
function polylang_slug_should_run( $query = '' ) {

	/**
	 * Disable front end query modification.
	 *
	 * Allows disabling front end query modification if not needed.
	 *
	 * @since 0.2.0
	 *
	 * @param bool     false  Not disabling run.
	 * @param WP_Query $query The WP_Query instance (passed by reference).
	 */

	// Do not run in admin or if Polylang is disabled
	$disable = apply_filters( 'polylang_slug_disable', false, $query );
	if ( is_admin() || is_feed() || ! function_exists( 'pll_current_language' ) || $disable ) {
		return false;
	}
	// The lang query should be defined if the URL contains the language
	$lang          = empty( $query->query['lang'] ) ? pll_current_language() : $query->query['lang'];
	// Checks if the post type is translated when doing a custom query with the post type defined
	$is_translated = ! empty( $query->query['post_type'] ) && ! pll_is_translated_post_type( $query->query['post_type'] );

	return ! ( empty( $lang ) || $is_translated );
}

/**
 * Standardize the query.
 *
 * This makes the standardized and simpler to run regex on
 *
 * @since 0.2.0
 *
 * @param  string $query Database query.
 *
 * @return string        The standardized query.
 */
function polylang_slug_standardize_query( $query ) {
	// Strip tabs, newlines and multiple spaces.
	$query = str_replace(
		array( "\t", " \n", "\n", " \r", "\r", "   ", "  " ),
		array( '', ' ', ' ', ' ', ' ', ' ', ' ' ),
		$query
	);
	return trim( $query );
}

/**
 * Fetch the polylang join clause.
 *
 * @since 0.2.0
 *
 * @return string
 */
function polylang_slug_model_post_join_clause() {
	if ( function_exists( 'PLL' ) ) {
		return PLL()->model->post->join_clause();
	} elseif ( array_key_exists( 'polylang', $GLOBALS ) ) {
		global $polylang;
		return $polylang->model->join_clause( 'post' );
	}
	return '';
}

/**
 * Fetch the polylang where clause.
 *
 * @since 0.2.0
 *
 * @param  string $lang The current language slug.
 *
 * @return string
 */
function polylang_slug_model_post_where_clause( $lang = '' ) {
	if ( function_exists( 'PLL' ) ) {
		return PLL()->model->post->where_clause( $lang );
	} elseif ( array_key_exists( 'polylang', $GLOBALS ) ) {
		global $polylang;
		return $polylang->model->where_clause( $lang, 'post' );
	}
	return '';
}