Unfortunately, EC Cube is not widely used outside of Japan and the English community is very small. The most we have is the plugin documentation (https://en.ec-cube.net/forum/plugin-guide)
and these support forums.
If you have specific questions, it's best to just ask them on these boards. Aside from that, just download the software, run it locally on xampp or vagrant box or something and just play around with it to learn!