The advantages of full-stack single-language web development are way overblown. Most developers (myself included) have absolutely no problem with using different languages between front and back ends. It’s been this way for years and years.
I favour using Amber Smalltalk on the front-end, but you could choose from any number of excellent transpiled alternatives, such as ClojureScript, Dart, Elm, Haxe, Scala.js, and Transcrypt (Python).
I favour Go and Python on the back end, but there’s a whole universe of great alternatives here, too. I see no reason why anyone should disregard the tried-and-true maxim: “Always choose the best tool for the job.”
I see no reason why anyone should choose a clearly inferior language just so you can avoid using two languages in the same software project. This nonsense of code sharing and not having to switch mental context between front and back ends is just that: nonsense.