Source

File: wp-admin/includes/class-plugin-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 plugin&#8230;' );
		$this->strings['remove_old']          = __( 'Removing the current plugin&#8230;' );
		$this->strings['remove_old_failed']   = __( 'Could not remove the current plugin.' );
		$this->strings['no_files']            = __( 'The plugin contains no files.' );
		$this->strings['process_failed']      = __( 'Plugin installation failed.' );
		$this->strings['process_success']     = __( 'Plugin installed successfully.' );
		/* translators: 1: Plugin name, 2: Plugin version. */
		$this->strings['process_success_specific'] = __( 'Successfully installed the plugin <strong>%1$s %2$s</strong>.' );

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

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