6 files declare their use of ConfigurableLanguageInterface in Drupal 9

block.module in core/modules/block/block.module

Controls the visual building blocks a page is constructed with.

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

LanguageTestController.php in core/modules/language/tests/language_test/src/Controller/LanguageTestController.php

locale.module in core/modules/locale/locale.module

Enables the translation of the user interface to languages other than English.

MigrateLanguageTest.php in core/modules/language/tests/src/Kernel/Migrate/d6/MigrateLanguageTest.php

node.module in core/modules/node/node.module

The core module that allows content to be submitted to the site.