Wrong subscriber's prefered language for anonymous user when interface language is not default

Results for the test case:

// Switch to non default language.
$this->drupalGet('/es');
// Subscibe.
$this->submitForm($edit, 'Subscribe');

$subscriber = $this->getLatestSubscriber();
// Expect to get non default language
$this->assertEquals('es', $subscriber->getLangcode(), 'Subscriber prefered language is set to user interface language');

For the moment developers can set language in the next way:

/**
 * Implements hook_form_FORM_ID_alter().
 *
 * Set anonymous subscriber's preferred language based on current user interface.
 * Fix Issue # 3447905.
 */
function mymodule_form_simplenews_subscriptions_block_[NEWSLETTER_ID]_alter(array &$form, FormStateInterface &$form_state): void {
  if (\Drupal::currentUser()->isAnonymous()) {
    array_unshift($form['actions']['submit']['#submit'], 'mymodule_alter_preferred_language');
  }
}

function mymodule_alter_preferred_language(array &$form, FormStateInterface $form_state): void {
  $form_state->setValue(['langcode', 0, 'value'], \Drupal::languageManager()->getCurrentLanguage()->getId());
}

Or even add language dropdown:

function mymodule_form_simplenews_subscriptions_block_[NEWSLETTER_ID]_alter(array &$form, FormStateInterface &$form_state): void {
  if (\Drupal::currentUser()->isAnonymous()) {
    array_unshift($form['actions']['submit']['#submit'], 'mymodule_alter_prefered_language');
    $form['lang'] = [
      '#type' => 'language_select',
      '#title' => t('Preferred language'),
      '#default_value' => \Drupal::languageManager()->getCurrentLanguage()->getId(),
      '#languages' => LanguageInterface::STATE_CONFIGURABLE,
    ];
  }
}

function mymodule_alter_prefered_language(array &$form, FormStateInterface $form_state): void {
  $selectedLang = $form_state->getValue('lang');
  $form_state->setValue(['langcode', 0, 'value'], $selectedLang ?? \Drupal::languageManager()->getCurrentLanguage()->getId());
}

UPD: for 4.x version the form hook is a bit different.

function mymodule_form_alter(array &$form, FormStateInterface &$form_state, $form_id): void {
  if ($form_id === 'simplenews_subscriptions_block_[SIMPLENEWS_ENTITY_UUID]') {
    //... Rest of the code from above.
  }
}

@Aditiup 16.92MB patch with over 234053 lines changed, is it purposely?

Regarding last commit in MR54 with +117/-168 changes in simplenews.module. These changes will never be reviewed/accepted at once.