Special system language code (only applicable to UI language).
Refers to the language used in Drupal and module/theme source code. Drupal uses the built-in text for English by default, but if configured to allow translation/customization of English, we need to differentiate between the built-in language and the English translation.
const LANGCODE_SYSTEM = 'system';
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https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Language!LanguageInterface.php/constant/LanguageInterface::LANGCODE_SYSTEM/8.1.x