/**
 * @param float[] $preferred
 * @return string
 */
protected function getDefaultLanguage( array $preferred ) {
	$supported = $this->languageNameUtils
		->getLanguageNames( LanguageNameUtils::AUTONYMS, LanguageNameUtils::SUPPORTED );

	// look for a language that is acceptable to the client
	// and known to the wiki.
	foreach ( $preferred as $code => $weight ) {
		if ( isset( $supported[$code] ) ) {
			return $code;
		}
	}

	// Some browsers might only send codes like de-de.
	// Try with bare code.
	foreach ( $preferred as $code => $weight ) {
		$parts = explode( '-', $code, 2 );
		$code = $parts[0];
		if ( isset( $supported[$code] ) ) {
			return $code;
		}
	}

	return '';
}