It is all about software, so the most optimal way to contribute is by coding.
Infrastructure and a well defined workflow is in place to handle code contributions, based on GitHub Pull Requests: https://github.com/ckeditor/ckeditor5/pulls.
Things to keep in mind:
Being this a project with global impact, contributing translations is both an easy and powerful way to help.
We use the Transifex service for translations at the following address: https://www.transifex.com/projects/p/ckeditor/.
Here as well, having a CLA in place is a requirement to become an official translator (see bellow).
The easiest way to contribute is by simply reporting issues that you find in our project. We use the following GitHub page to track and manage them: https://github.com/ckeditor/ckeditor5/issues.
Things to keep in mind:
The issues pages can be used for feature requests.
To accept contributions sent to us in form of code, documentation or translations, a Contributor License Agreement (CLA) must be in place in order to clarify the intellectual property license granted with them. This license is for your protection as a contributor as well as the protection of us and our users; it does not change your rights to use your own contributions for any other purpose.
To sign the CLA and to have more information, please follow this link: http://ckeditor.com/cla.