It seems like the intention is to use Flutter [1] as the UI layer. Flutter uses the Dart language, so there's a Dart environment included in Fuchsia too [2].
For rendering, Fuchsia includes a project called Escher [3] which is described as a physically based renderer that supports soft shadows, light diffusion and other advanced effects. Looking at the source code, Escher is designed to use either OpenGL or Vulkan as the underlying graphics API. (There's an iOS example project included in Escher's source tree. Would be interesting to build that.)
It's not immediately obvious why a lightweight operating system would need a renderer that can do realtime soft shadows and light effects...! But I think the idea here is to build an UI layer that's designed from scratch for Google's Material design language. Shadows and subtle color reflections are a major part of that "layered paper" aesthetic.
So, the stack seems to be: Dart is the language for GUI apps, Flutter provides the widgets, and Escher renders the layers.
The underlying application framework is called Mojo [4]. It already offers bindings for Go, Java, JavaScript, Python and Rust in addition to Dart, but maybe those languages are meant for services rather than GUI apps. (At least I can't see an easy way to create Flutter widgets from something like Rust without loading the Dart VM.)
[2] https://fuchsia.googlesource.com/dart_content_handler/