Source

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

public function upgrade_strings() {
		$this->strings['up_to_date'] = __( 'The theme is at the latest version.' );
		$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['remove_old']          = __( 'Removing the old version of the theme&#8230;' );
		$this->strings['remove_old_failed']   = __( 'Could not remove the old theme.' );
		$this->strings['process_failed']      = __( 'Theme update failed.' );
		$this->strings['process_success']     = __( 'Theme updated successfully.' );
	}