LanguageManager class is the wrong class to use, since it is defined as follows.
Class responsible for providing language support on language-unaware sites.
Given that description, and the fact there is a service using that class, the correct code should instantiate a service, which is what the code in install.core.inc does. (See
// Determine whether a translation file must be imported during the
// 'install_import_translations' task. Import when a non-English language is
// available and selected. Also we will need translations even if the
// installer language is English but there are other languages on the system.
$needs_translations = (count($install_state['translations']) > 1 && !empty($install_state['parameters']['langcode']) && $install_state['parameters']['langcode'] != 'en') || \Drupal::languageManager()->isMultilingual();
So, the correct code to use in procedural code is the following.
$is_multi_lingual = \Drupal::languageManager()->isMultilingual();
When using a class, and when it is possible, the service should be injected, and it instance saved in a object property (e.g.
$this->languageManager). In that case, the code is the following.
$is_multi_lingual = $this->languageManager->isMultilingual();
The service ID for the language manager is language_manager.