<?php
include_once('wp-load.php');

$args = array(
    'posts_per_page' => -1,
    'post_type' => 'post',
    'lang' => 'pl',
);

$query = new WP_Query( $args );

$i = 0;

if ( $query->have_posts() ) {
    $posts_data = array();

    while ( $query->have_posts() ) {
        $i++;
        $query->the_post();

        $post_data = array(
            'id' => $i,
            'title' => get_the_title(),
            'url' => get_permalink(),
        );

        $posts_data[] = $post_data;
    }

    header('Content-Type: application/json');
    echo json_encode($posts_data, JSON_UNESCAPED_UNICODE | JSON_PRETTY_PRINT);

    wp_reset_postdata(); 
} else {
    echo json_encode(array('error' => 'not found.'));
}