The source code of the free version (GPLv2+ license) can be found in a public GitHub repository.

Any help in improving the Plugin is truly appreciated. If you would like to contribute in development you need to be familiar with Git(Hub), WordPress Coding Standards and modern WordPress development (i.e. Gutenberg).

Translations

You can also help translating the Plugin into different languages by selecting the “Help translate” menu item (them.es user account required) or by contributing via https://translate.wordpress.org (WordPress.org user account required).