Environment::getLanguageFallbackChain() 

\nn\t3::Environment()->getLanguageFallbackChain($langUid = true); 

Returns a list of the languages that should be used if, for example e.g. a page or element does not exist in the desired language.

Important: The fallback chain contains in the first place the current or in $langUid transferred language.

// Use settings for current language (see Site-Config YAML)
\nn\t3::Environment()->getLanguageFallbackChain(); // --> e.g. [0] or [1,0]

// Get settings for a specific language
\nn\t3::Environment()->getLanguageFallbackChain( 1 );
// --> [1,0] - if fallback was defined in Site-Config and the fallbackMode is set to "fallback"
// --> [1] - if there is no fallback or the fallbackMode is set to "strict"
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@param string|boolean $returnKey
@return string|array

Source Code 

public function getLanguageFallbackChain( $langUid = true )
{
	if ($langUid === true) {
		$langUid = $this->getLanguage();
	}
	$langSettings = $this->getLanguages()[$langUid] ?? [];
	$fallbackType = $langSettings['fallbackType'] ?? 'strict';
	$fallbackChain = $langSettings['fallbacks'] ?? '';
	if ($fallbackType == 'strict') {
		$fallbackChain = '';
	}
	$fallbackChainArray = array_map( function ( $uid ) {
		return intval( $uid );
	}, \nn\t3::Arrays($fallbackChain)->intExplode() );
	array_unshift( $fallbackChainArray, $langUid );
	return $fallbackChainArray;
}
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