There are really two different kinds of developers. Those that want to embed Daffodil into an application, and those who want to contribute to Daffodil itself. These are not mutually exclusive.
Embedding Daffodil in an Application
If you want to embed Daffodil into some other system so that you can parse data using DFDL schemas to describe the format, then you can develop in Java or in Scala. There are APIs for both.
Developing Daffodil Itself
If you want to help in the development of Daffodil itself, then you will need to know or learn Scala, because that's what it is written in.
now an Apache Incubator project
For users, see the Apache Daffodil (incubating) web site
For developers, see the Apache Daffodil (incubating) wiki