14 files declare their use of LanguageNegotiationUrl in Drupal 9

BlockUiTest.php in core/modules/block/tests/src/Functional/BlockUiTest.php

BookMultilingualTest.php in core/modules/book/tests/src/Kernel/BookMultilingualTest.php

EntityUrlLanguageTest.php in core/modules/language/tests/src/Kernel/EntityUrlLanguageTest.php

language.module in core/modules/language/language.module

Add language handling functionality to Drupal.

LanguageNegotiationContentEntityTest.php in core/modules/language/tests/src/Functional/LanguageNegotiationContentEntityTest.php

LanguageNegotiationUrlTest.php in core/modules/language/tests/src/Unit/LanguageNegotiationUrlTest.php

LanguageSwitchingTest.php in core/modules/language/tests/src/Functional/LanguageSwitchingTest.php

LanguageUILanguageNegotiationTest.php in core/modules/language/tests/src/Functional/LanguageUILanguageNegotiationTest.php

LanguageUrlRewritingTest.php in core/modules/language/tests/src/Functional/LanguageUrlRewritingTest.php

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

MigrateLanguageNegotiationSettingsTest.php in core/modules/language/tests/src/Kernel/Migrate/d7/MigrateLanguageNegotiationSettingsTest.php

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

NodeFieldMultilingualTest.php in core/modules/node/tests/src/Functional/NodeFieldMultilingualTest.php

PathProcessorTest.php in core/tests/Drupal/Tests/Core/PathProcessor/PathProcessorTest.php