This functions returns a WordPress page permalink for the current language by its slug.

polylang.php

<?php

/**
 * This function returns a page permalink
 * for the current website language.
 *
 * @author  Mauricio Gelves <mg@maugelves.com>
 * @param   $page_slug      string          WordPress page slug
 * @return                  string|false    Page Permalink or false if the page is not found
 */
function pll_get_page_url( $page_slug ) {

	// Check parameter
	if( empty( $page_slug ) ) return false;
	
	// Get the page
	$page = get_page_by_path( $page_slug );

	// Check if the page exists
	if( empty( $page ) || is_null( $page ) ) return false;

	// Get the URL
	$page_ID_current_lang = pll_get_post( $page->ID );

	// Return the current language permalink
	return empty($page_ID_current_lang) ? get_permalink( $page->ID ) : get_permalink( $page_ID_current_lang );

}