custom block field not translatable

i have created a custom block

<?php

namespace Drupal\amu_external_search_block\Plugin\Block;

use Drupal\Core\Block\BlockBase;
use Drupal\Core\Block\BlockPluginInterface;
use Drupal\Core\Form\FormStateInterface;

/**
 * Provides a 'amu_external_search_form_block' block.
 *
 * @Block(
 *  id = "amu_external_search_form",
 *  admin_label = @Translation("recherche externe"),
 * )
 */
class AmuSearchFormBlock extends BlockBase implements BlockPluginInterface {
  /**
   * {@inheritdoc}
   */
  public function build() {
    $form = \Drupal::formBuilder()
      ->getForm('Drupal\amu_external_search_block\Form\AmuSearchForm');
    $content = $form;
    $elements[] = [
      '#theme' => 'amu_external_search_block',
      '#content' => $content,
    ];
    return $elements;
  }

  /**
   * Ajoute à l'instanciation du block un form input pour configurer l'url du
   * submit button.
   *
   * {@inheritdoc}
   */
  public function blockForm($form, FormStateInterface $form_state) {
    $form = parent::blockForm($form, $form_state);
    $config = $this->getConfiguration();
    $form['submit_action_amu_external_search_form_block_settings'] = [
      '#type' => 'textfield',
      '#title' => $this->t('URL de recherche externe'),
      '#default_value' => isset($config['submit_action_amu_external_search_form_block_settings']) ? $config['submit_action_amu_external_search_form_block_settings'] : 'https://univ-amu.summon.serialssolutions.com/#!/search?ho=t&l=fr-FR&q=',
    ];
    $form['amu_external_search_block_description'] = [
      '#type' => 'textarea',
      '#title' => $this->t('Complément d\'information'),
      '#default_value' => isset($config['amu_external_search_block_description']) ? $config['amu_external_search_block_description'] : '',
      '#description' => 'Ajout d\'informations ou d\'un lien (code html optionnel)'
    ];
    $form['amu_external_search_block_placeholder'] = [
      '#type' => 'checkbox',
      '#title' => $this->t('Afficher le placeholder'),
      '#default_value' => isset($config['amu_external_search_block_placeholder']) ? $config['amu_external_search_block_placeholder'] : false,
    ];
    $form['amu_external_search_block_title'] = [
      '#type' => 'checkbox',
      '#title' => $this->t('Afficher l\'étiquette de la barre de recherche externe'),
      '#default_value' => isset($config['amu_external_search_block_title']) ? $config['amu_external_search_block_title'] : true,
    ];
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function blockSubmit($form, FormStateInterface $form_state) {
    $this->configuration['submit_action_amu_external_search_form_block_settings'] = $form_state->getValue('submit_action_amu_external_search_form_block_settings');
    $this->configuration['amu_external_search_block_description'] = $form_state->getValue('amu_external_search_block_description');
    $this->configuration['amu_external_search_block_placeholder'] = $form_state->getValue('amu_external_search_block_placeholder');
    $this->configuration['amu_external_search_block_title'] = $form_state->getValue('amu_external_search_block_title');
  }
}

How to make the textfield and textarea input translatable using the block translation from the back office ?