drupal-reha/web/modules/reha_mod/reha_mod.module

<?php

/**
 * @file
 * Primary module hooks for reha module.
 */


function reha_mod_form_user_register_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  $form['actions']['submit']['#value'] = "valider";
}