Issue #2153937 by Gábor Hojtsy, pcambra: Default language setting is hard to find

core/modules/block/src/Tests/BlockLanguageTest.php

.../config_translation/src/Tests/ConfigTranslationUiTest.php

core/modules/language/language.module

core/modules/language/src/LanguageListBuilder.php

.../modules/language/src/Tests/LanguageConfigurationTest.php

core/modules/language/src/Tests/LanguageListTest.php

...odules/language/src/Tests/LanguagePathMonolingualTest.php

core/modules/locale/src/Tests/LocaleContentTest.php

core/modules/node/src/Tests/NodeTypeInitialLanguageTest.php

core/modules/search/src/Tests/SearchLanguageTest.php