I've seen Kotlin - few issues :
* it isn't 1.0 yet (guaranteed to have breaking changes)
* I don't see it as a dynamic/prototyping language - I see it more as a C# than as Python, frankly for the front end code that I want to write the python style of development is much faster and Groovy seems to have an answer if the performance of dynamic typing ends up being an issue. Most of the complex datastractures are in C++ anyway and will have a simple/clean interface to the front end
* it's an extra language to learn - not only me but by my coworkers as well. We already took on Groovy with Grails and we have a decent amount of build automation in it (and will have more)
I've learned a lot of languages so far (C++, Python, C#, Java, Clojure, JavaScript and I've played with Rust, F#, TypeScript, Objective C) - frankly the value of having as few languages as possible to share the actual code/tools/knowledge between different areas is huge and in order to introduce a new language to the mix it better have a significant benefit over the ones already used.