microCMS is a REST API-based CMS. It have simple user interface and functions for Japanese.
Have many types for content. text, date, boolean, rich editor, reference and more...
You can preview contents by "drafyKey". It ’s a very simple mechanism so it ’s easy to understand.
Imgix integration is included.
It's have Slack and Netlify Webhooks integrations.
Find more headless content management systems.