Convert::toLanguageId() 

\nn\t3::Convert()->toLanguageId($languageCode = NULL); 

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

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

Source Code 

public function toLanguageId($languageCode = null)
{
	if (is_numeric($languageCode)) {
		return $languageCode;
	}
	$key = $languageCode !== null ? $languageCode : $this->initialArgument;
	$languages = \nn\t3::Environment()->getLanguages('iso-639-1', 'languageId');
	return $languages[$key] ?? '';
}
Copied!