AFAIK the flutter UI framework is a react-like framework written in Dart (with C++ as OS glue) including the UI -> graphics rendering layer. It builds upon Skia and Blink. I am not sure how that will allow compatibility with other languages. The only language for UI apps looks to be Dart. Which isn't bad - its a pretty well designed language, but I don't see how apps can be written in a wide variety of language as the author suggests.