installLanguages example

// Create and log in as admin.     $this->adminUser = $this->drupalCreateUser([
      'administer languages',
      'access administration pages',
      'administer site configuration',
      'translate interface',
    ]);
    $this->drupalLogin($this->adminUser);

    // Install languages.     $this->installLanguages();

    // Assign Lolspeak (xx) to be the default language.     $this->config('system.site')->set('default_langcode', 'xx')->save();
    $this->rebuildContainer();

    // Check that lolspeak is the default language for the site.     $this->assertEquals('xx', \Drupal::languageManager()->getDefaultLanguage()->getId(), 'Lolspeak is the default language');
  }

  /** * Tests Twig "trans" tags. */