mlp-api-extension.php

<?php
/**
 * Plugin Name: MultilingualPress REST API Extension
 * Description: Adds REST endpoints that allow users to get translations for a specific post or term.
 * Version:     0.1
 */

class MLP_REST_API_Extension {

  public function __construct() {
    add_action( 'rest_api_init', array( $this, 'register_api_routes' ) );
  }

  /**
  * register api routes
  */
  public function register_api_routes() {

    register_rest_route( 'mlp/v1', '/posts/(?P<id>\d+)', array(
	'methods' => 'GET',
	'callback' => array( $this, 'get_post_translations' ),
    ) );

  }

  /**
  * Get post translations based on source post id
  * @uses mlp_get_linked_elements in multilingual-press 2.5.4 inc / functions.php
  * @uses mlp_get_blog_language in multilingual-press 2.5.4 inc / functions.php
  * @param object
  * @return array
  */
  public function get_post_translations( $data ) {

    $translations = array();
    $current_blog_id = get_current_blog_id();

    /*
    gets an array with linked elements relation
    where key is the blog id and value is the post id, example:
    {
      "1": 5,
      "2": 7,
      "4": 5
    }
    */
    $linked_posts = mlp_get_linked_elements( $data['id'] );

    // loop through $linked_posts and populates $translations array
    foreach ( $linked_posts as $key => $value ) {

      switch_to_blog( $key );

      $arr = array(
        'blog_id' => $key,
        'post_id' => $value,
        'locale' => mlp_get_blog_language( $key, FALSE ),
        'title' => get_the_title( $value ),
        'content' => apply_filters( 'the_content', get_post_field( 'post_content', $value ) )
      );

      array_push( $translations, $arr );

    }

    switch_to_blog( $current_blog_id );

    return $translations;

  }

}
new MLP_REST_API_Extension();