10 files declare their use of LanguageManager in Drupal 9

ActiveLinkResponseFilterTest.php in core/tests/Drupal/Tests/Core/EventSubscriber/ActiveLinkResponseFilterTest.php

ConfigurableLanguage.php in core/modules/language/src/Entity/ConfigurableLanguage.php

ConfigurableLanguageManager.php in core/modules/language/src/ConfigurableLanguageManager.php

install.core.inc in core/includes/install.core.inc

API functions for installing Drupal.

InstallerLanguagePageTest.php in core/tests/Drupal/FunctionalTests/Installer/InstallerLanguagePageTest.php

Language.php in core/modules/ckeditor/src/Plugin/CKEditorPlugin/Language.php

LanguageAddForm.php in core/modules/language/src/Form/LanguageAddForm.php

LanguageTest.php in core/modules/ckeditor/tests/src/Unit/Plugin/CKEditorPlugin/LanguageTest.php

SelectLanguageForm.php in core/lib/Drupal/Core/Installer/Form/SelectLanguageForm.php

UserLangcode.php in core/modules/user/src/Plugin/migrate/process/UserLangcode.php