Having said that, developing and maintaining such an application in the real world quickly escalates into a proverbial fireball that gets tangled in the different parts of the application. Now let me explain this - for instance, a part of team is writing a web-service in java, and another part is writing a different part in python. This leaves with no scope of inter-operability in future (if one needs to mix and match say 2 functions of these two services). I can also cite example of Ruby On Rails into this, where the fairly immaculate ruby code gets interspersed with bloated html code, which leaves a bad taste in the maintainability of the application.
A wonderful example is the case of node.js which has proven why a fairly consistent language helps alleviate issues of managing different languages in a single application, thereby creating fewer bugs and reducing the overall development coordination and time.
I've found scalaJs to be an equally good tool to be used in Play/ scala based web applications and services which require complex frontend development as well. Hopefully, I'll be able to write some applications to try and assess its impact over conventional apps.