6 files declare their use of ConfigurableLanguageInterface

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.

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