Drupal - Migrate existing field content from language "undefined" to entity language in a single hook_update_N() implementation.

MODULE.install.php

<?php

/**
 * Migrate existing field content from language "undefined" to entity language.
 */
function MODULE_update_N(&$sandbox) {
  // Number of entities to be processed for each step.
  $messages = array();
  if (!isset($sandbox['fields'])) {
    // Initialize the array of field to process.
    $sandbox['fields'] = array_filter(field_info_fields(), function($field_info) {return $field_info['translatable'];});
    foreach (array_keys($sandbox['fields']) as $field_name) {
      $sandbox['fields'][$field_name]['context'] = array('sandbox' => array(), 'finished' => 0);
    }
  }
  // Search for an unfinished field.
  $field_names = array_keys($sandbox['fields']);
  $field_name = &reset($field_names);
  while ($field_name && $sandbox['fields'][$field_name]['context']['finished'] >= 1) {
    $field_name = &next($field_names);
  }
  if ($field_name) {
    // Process the field.
    module_load_include('inc', 'entity_translation', 'entity_translation.admin');
    entity_translation_translatable_batch(TRUE, $sandbox['fields'][$field_name]['field_name'], TRUE, $sandbox['fields'][$field_name]['context']);
    $sandbox['#finished'] = array_reduce($sandbox['fields'], function($carry, $item) {
        return $carry + $item['context']['finished'];
      }, 0) / count($sandbox['fields']);
    $message_args = array(
      '@field_name' => $sandbox['fields'][$field_name]['field_name'],
      '@progress' => (new NumberFormatter('en_US', NumberFormatter::PERCENT))->format($sandbox['#finished']),
      '@field_progress' => (new NumberFormatter('en_US', NumberFormatter::PERCENT))->format($sandbox['fields'][$field_name]['context']['finished'])
    );
    if ($sandbox['fields'][$field_name]['context']['finished'] >= 1) {
      $messages[] = format_string('@progress - Done with @field_name.', $message_args);
    }
    else {
      $messages[] = format_string('@progress - Processing @field_name (@field_progress)... ', $message_args);
    }
  }
  else {
    // No more field to process, we are done.
    $sandbox['#finished'] = 1;
  }
  return implode("\n", $messages);
}