One of my customers uses Siebel CRM. They've asked me to work on a custom UI, but I don't specialise in Java.
Is it possible to somehow use the interface that Siebel Open UI uses (in for example JSON) to create a separate UI?
Java is not a required skill for customizing Siebel's Open UI framework. The required skills are JavaScript, HTML, and CSS. The framework already provides complete flexibility for creating a custom user interface.