With the given info, the only issues which i can guess of is,
- The template cannot be used under the configured path.(Check for the allowedPaths property and see if it is ok to use with your configured path).
- The name property for your textarea i.e., "It's my jcr:content" field. Make sure you specify the name as a relative path from the page node.
i.e., in case you want to store it as a property called "foo" in "jcr:content" then provide the name as "./jcr:content/foo" and not as "./foo"(which is how we generally provide the value for the property "name" for widgets in the components dialog). Else ConstraintViolationException may be thrown.
Even you find that the issue is still there, then check the network tab in devtools(F12) and provide the response for the POST request to /content/home/en/*