Drupal 8: How to load available translations of a given node, change some fields and save.

load_node_translations.php

<?php

// Credit to Gábor Hojtsy
// http://hojtsy.hu/blog/2015-nov-11/drupal-8-multilingual-tidbits-19-content-translation-development

use Drupal\node\Entity\Node;

// Load node 4. In terms of language, this will get us an entity
// in the original submission language.
$node = Node::load(4);

// Get a list of all translations and collect titles.
$titles = [];
$languages = $node->getTranslationLanguages();
foreach ($languages as $langcode => $language) {
  // The object returned by getTranslation() behaves the same way as $node.
  $translation = $node->getTranslation($langcode);
  $titles[$langcode] = $translation->title;
}

// If the node has no Hungarian translation, add one.
if (!$node->hasTranslation('hu')) {
  $translation = $node->addTranslation('hu', array('title' => 'Hungarian title'));
  $translation->save();
}

// If the node has a Spanish translation, update the title. In case of a missing
// Spanish translation this will throw an InvalidArgumentException.
$node->getTranslation('es')->setTitle('Spanish title')->save();

// Remove the Hungarian translation.
$node->removeTranslation('hu');
$node->save();

?>