Issue #2609256 by sleitner, rkoller, smustgrave, vegantriathlete, quietone,...

    \Drupal::configFactory()->getEditable('locale.settings')->set('translation.path', $directory)->save();

  }

  \Drupal::service('file_system')->prepareDirectory($directory, FileSystemInterface::CREATE_DIRECTORY | FileSystemInterface::MODIFY_PERMISSIONS);

  $t_args = [

    ':translate_status' => base_path() . 'admin/reports/translations/check?destination=' . urlencode(base_path() . 'admin/reports/translations'),

  ];

  $message = t('Check <a href=":translate_status">available translations</a> for your language(s).', $t_args);

  \Drupal::messenger()->addStatus($message);

}

/**

<?php

declare(strict_types=1);

namespace Drupal\Tests\locale\Functional;

use Drupal\Tests\BrowserTestBase;

/**

 * Test installation of Locale module.

 *

 * @group locale

 */

class LocaleInstallTest extends BrowserTestBase {

  /**

   * {@inheritdoc}

   */

  protected static $modules = [

    'system',

    'file',

    'language',

  ];

  /**

   * {@inheritdoc}

   */

  protected $defaultTheme = 'stark';

  /**

   * Tests Locale install message.

   */

  public function testLocaleInstallMessage(): void {

    $admin_user = $this->drupalCreateUser([

      'access administration pages',

      'administer modules',

    ]);

    $this->drupalLogin($admin_user);

    $edit = [];

    $edit['modules[locale][enable]'] = 'locale';

    $this->drupalGet('admin/modules');

    $this->submitForm($edit, 'Install');

    $this->assertSession()->statusMessageContains('available translations', 'status');

  }

}