Drupal - How do I get a field translated in Drupal 8

You can get a specific translation with $translation = $entity->getTranslation('de'), then you can use $translation exactly the same as $entity.

Usually, you don't want to get a specific translation, but the best translation that is available for the current language, for that, use getTranslationFromContext():

 $translation = \Drupal::service('entity.repository')->getTranslationFromCon‌​text($entity);

Loading all help_page and return it in the current language:

$elements = [];
$ids = \Drupal::entityQuery('node')
  ->condition('type', 'help_page')
  ->execute();
$nodes = Node::loadMultiple($ids);
$language = \Drupal::languageManager()->getCurrentLanguage()->getId();

foreach ($nodes as $node) {
  $translation = $node->getTranslation($language);
  $elements[] = [
    'title' => $translation->title->value,
    'url' => $translation->url(),
    'img' => $translation->field_imagen->entity->url(),
    'text' => $translation->body->value,
  ];
}
return $elements;