Source

File: wp-admin/includes/class-language-pack-upgrader.php

public function upgrade( $update = false, $args = array() ) {
		if ( $update ) {
			$update = array( $update );
		}

		$results = $this->bulk_upgrade( $update, $args );

		if ( ! is_array( $results ) ) {
			return $results;
		}

		return $results[0];
	}