I am not averse to using multiple languages in a project if each serves a need so, in the past two months, I have used gsl, TCL, Python, C, and Go for various components of an ongoing project. But not necessarily all at the same time. Most days, I focus on one and only deal with two simultaneously during integration or code generation.
I've been leaning towards full-stack JS as much as possible. If only that it makes some of the communication channels much easier to work across. There's still templating (html/jade/mustache/markdown) and styling (css/less/sass/stylus) to consider, and not mentioning interaction with other systems/services (mongodb/sql/salesforce). Bringing as much of that under one umbrella as possible is useful.
On the flip side, if you have well documented services, and workers/queues you can break your load up and use a lot of different platforms.
Also, "platform spread" is a real concern.