What in the world would make you think they'd use C++ or C# for a UI addon framework? Are there even any C# UI addon libraries out there? (Legitimate question, I'm a C# fanboy but I can't even remotely imagine game addon UI as a use case for the language.)

Anyways, they've said they're using Flash, which is actually -amazing- for game UI work from everything I've ever read about the issue; I was hoping for Lua since I'm more familiar with it personally but it's not like it'll be hard to learn Flash.