It will probably be LUA only.
I don't know if the in-game customizable screens will be in HTML. And I hope not. I mean it's so cheap, there's no interest if it's not for an in-game net browser imo.
Surely some hard coded customizable thing would be better (LUA or even without necessary coding for the layout that would be NEAT).
IMO:
JS is quite flexible but really lacks orthography. I mean come on no one can read it but a mathematician, it just doesn't look easy to work with. Plus it's sometimes "too flexible" as it will let you do errors that can lead the project to be full of it (cool debugging afterwards...).
C# is just bliss... it's C++ but easier, and is good to begin with, but really less powerful and flexible than C++ that is more customizable. But it also let errors pass the net, so you have some debugging issues where you can't find where the code leak is, or why it occurs.
C++ let practically no error passing by, they are all detected on build, but that means more work, and is not needed for a small project.
So basically in DU the best would be LUA embedded. JS is for the web and web games sometimes, even though it's not the only option for web games, and I prefer other solutions.
Long live little LUA and compiled languages.