override-example-at-runtime.php

<?php

// Remember original language before this operation.
$original_language = \Drupal::configFactory()->getLanguage();

$language = language_load($account->getPreferredLangcode());
// Set the languag without saving, this is actually only for the current script execution:
\Drupal::configFactory()->setLanguage($language);
// Get the user.mail config object normally, but for the language we just set :)
$mail_config = \Drupal::config('user.mail');

// Now send email based on $mail_config which is in the proper language.
//...

// Set back the language to the prior value.
\Drupal::configFactory()->setLanguage($original_language);

?>