Convert::toLanguage() 

\nn\t3::Convert()->toLanguage($sysLanguageUid = NULL); 

Converts a language ID (e.g. '0', '1') into the two-character language abbreviation (e.g. 'de', 'en')

// Language ID 0 -> 'de'
\nn\t3::Convert(0)->toLanguage();
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@param int $sysLanguageUid
@return string

Source Code 

public function toLanguage($sysLanguageUid = null)
{
	if (is_string($sysLanguageUid) && strlen($sysLanguageUid) == 2) {
		return $sysLanguageUid;
	}
	$key = $sysLanguageUid !== null ? $sysLanguageUid : $this->initialArgument;
	$languages = \nn\t3::Environment()->getLanguages('languageId', 'iso-639-1');
	return $languages[$key] ?? '';
}
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