The language code used when no language is explicitly assigned (yet).
Should be used when language information is not available or cannot be determined. This special language code is useful when we know the data might have linguistic information, but we don't know the language.
See http://www.w3.org/International/questions/qa-no-language#undetermined.
const LANGCODE_NOT_SPECIFIED = 'und';
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https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Language!LanguageInterface.php/constant/LanguageInterface::LANGCODE_NOT_SPECIFIED/8.1.x