After some research we narrowed it down to Scala and Go, and eventually decided to go with Go! Scala has excellent Play! framework, but Go feels more like Python
For Mongo: Most people use mgo http://labix.org/mgo
For precog: dont know, but there might be something, but probably not.
I just forgot to say: a FRP* [0] Go equivalent of Play and that there's nothing like these upcoming Coursera courses [1] [2] for Go with examples.
[0] http://www.reactivemanifesto.org