Environment::getDefaultLanguage() 

\nn\t3::Environment()->getDefaultLanguage($returnKey = 'typo3Language'); 

Returns the default language (Default Language). In TYPO3, this is always the language with ID 0 The languages must be defined in the YAML site configuration.

// 'de'
\nn\t3::Environment()->getDefaultLanguage();

// 'de-DE'
\nn\t3::Environment()->getDefaultLanguage('hreflang');

// ['title'=>'German', 'typo3Language'=>'de', ...]
\nn\t3::Environment()->getDefaultLanguage( true );

public function getDefaultLanguage( $returnKey = 'typo3Language' ) {
	$firstLanguage = $this->getLanguages('languageId')[0] ?? [];
	if ($returnKey === true) return $firstLanguage;
	return $firstLanguage[$returnKey] ?? '';
}