Source

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

public function install_strings() {
		$this->strings['no_package'] = __( 'Installation package not available.' );
		/* translators: %s: Package URL. */
		$this->strings['downloading_package'] = sprintf( __( 'Downloading installation package from %s&#8230;' ), '<span class="code">%s</span>' );
		$this->strings['unpack_package']      = __( 'Unpacking the package&#8230;' );
		$this->strings['installing_package']  = __( 'Installing the theme&#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['no_files']            = __( 'The theme contains no files.' );
		$this->strings['process_failed']      = __( 'Theme installation failed.' );
		$this->strings['process_success']     = __( 'Theme installed successfully.' );
		/* translators: 1: Theme name, 2: Theme version. */
		$this->strings['process_success_specific'] = __( 'Successfully installed the theme <strong>%1$s %2$s</strong>.' );
		$this->strings['parent_theme_search']      = __( 'This theme requires a parent theme. Checking if it is installed&#8230;' );
		/* translators: 1: Theme name, 2: Theme version. */
		$this->strings['parent_theme_prepare_install'] = __( 'Preparing to install <strong>%1$s %2$s</strong>&#8230;' );
		/* translators: 1: Theme name, 2: Theme version. */
		$this->strings['parent_theme_currently_installed'] = __( 'The parent theme, <strong>%1$s %2$s</strong>, is currently installed.' );
		/* translators: 1: Theme name, 2: Theme version. */
		$this->strings['parent_theme_install_success'] = __( 'Successfully installed the parent theme, <strong>%1$s %2$s</strong>.' );
		/* translators: %s: Theme name. */
		$this->strings['parent_theme_not_found'] = sprintf( __( '<strong>The parent theme could not be found.</strong> You will need to install the parent theme, %s, before you can use this child theme.' ), '<strong>%s</strong>' );
		/* translators: %s: Theme error. */
		$this->strings['current_theme_has_errors'] = __( 'The current theme has the following error: "%s".' );

		if ( ! empty( $this->skin->overwrite ) ) {
			if ( 'update-theme' === $this->skin->overwrite ) {
				$this->strings['installing_package'] = __( 'Updating the theme&#8230;' );
				$this->strings['process_failed']     = __( 'Theme update failed.' );
				$this->strings['process_success']    = __( 'Theme updated successfully.' );
			}

			if ( 'downgrade-theme' === $this->skin->overwrite ) {
				$this->strings['installing_package'] = __( 'Downgrading the theme&#8230;' );
				$this->strings['process_failed']     = __( 'Theme downgrade failed.' );
				$this->strings['process_success']    = __( 'Theme downgraded successfully.' );
			}
		}
	}