Drupal 8 - Programmatically translate strings via hook_update_N()

add_translations.install

/**
 * Helper function for adding translations.
 *
 * @param array $strings
 *   The array with strings and their translations.
 */
function _my_module_add_translations(array $strings) {

  // Get locale storage service.
  $storage = \Drupal::service('locale.storage');

  // Loop through the translations.
  foreach ($strings as $translation) {

    // Find the source string.
    try {
      $string = $storage->findString(['source' => $translation['label']]);

      if (is_null($string)) {
        $string = new SourceString();
        $string->setString($translation['label']);
        $string->setStorage($storage);
        $string->save();
      }

      // Create translation. If one already exists, it will be replaced.
      foreach ($translation['translations'] as $langcode => $string_translation) {
        $storage->createTranslation([
          'lid' => $string->lid,
          'language' => $langcode,
          'translation' => $string_translation,
        ])->save();
      }
    }
    catch (Exception $e) {
      \Drupal::logger('my_module')->warning('Error trying to find string @s: @e on line @l', [
        '@s' => $translation['label'],
        '@e' => $e->getMessage(),
        '@l' => $e->getLine(),
      ]);
    }
  }
}

/**
 * Add translations.
 */
function my_module_update_8001() {

  // Add the translations.
  $translations = [
    [
      'label' => 'An example',
      'translations' => [
        'nl' => 'Een voorbeeld',
      ],
    ],
  ];

  // Add the translations.
  _my_module_add_translations($translations);
}