There is information here for DFDL Users, as well as for Daffodil Developers.
(Under construction, but there will be links here to getting started guide, some DFDL introduction materials, downloads for a simple command-line based Daffodil processor that takes in data and spits out XML. Also tutorials, how-to report a bug, FAQ, etc.)
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.
(Under construction, but links will be for downloading the jars, online javadoc/scaladoc, examples, etc.)
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.
(Under construction, but there will be links here to the pages that show you how to check-out, build, and run Daffodil as a developer using the Scala programming language, some of our project workflow practices, some Scala idioms we use, and so forth.)