Source

File: wp-admin/includes/class-core-upgrader.php

public function upgrade_strings() {
		$this->strings['up_to_date'] = __( 'WordPress is at the latest version.' );
		$this->strings['locked']     = __( 'Another update is currently in progress.' );
		$this->strings['no_package'] = __( 'Update package not available.' );
		/* translators: %s: Package URL. */
		$this->strings['downloading_package']   = sprintf( __( 'Downloading update from %s&#8230;' ), '<span class="code">%s</span>' );
		$this->strings['unpack_package']        = __( 'Unpacking the update&#8230;' );
		$this->strings['copy_failed']           = __( 'Could not copy files.' );
		$this->strings['copy_failed_space']     = __( 'Could not copy files. You may have run out of disk space.' );
		$this->strings['start_rollback']        = __( 'Attempting to roll back to previous version.' );
		$this->strings['rollback_was_required'] = __( 'Due to an error during updating, WordPress has rolled back to your previous version.' );
	}