I've tried working in an FP style in the "multiparadigm" languages you've mentioned, but it's hard, really hard. Well actually, the irony is that JavaScript is the one that's bearable and that's because (1) you can find some Javascript libraries that are doing FP and (2) it's very flexible.

Static languages such as Java and C# are not OK though. For one they lack features that are essential for statically typed languages. For example higher kinded types is an essential feature for FP. Without that you can't build libraries of reusable FP stuff. Comparing it with Scala for example you can't build Scalaz or the newer Cats or Shapeless [1].

The other problem is that FP programming languages come with FP libraries. You can't underestimate the importance of having good implementations for persistent collections be the default.

On having "no side effects", that's not what FP is about. After all, without side-effects you couldn't get any output out of a program. FP is about encoding those side-effects in types and abstractions you can reason about, or in other words it's about controlling side-effects. Haskell developers are fond of their IO monads. FRP is another way of doing that, being particularly good at dealing with user interfaces. And for example FRP-related libraries that were born by playing to the strengths of the mainstream languages we are dealing with are Facebook's React and Microsoft's Rx.

BTW, this is one of the best books on functional programming available today: http://www.manning.com/bjarnason/

[1] https://github.com/scalaz/scalaz, https://github.com/non/cats, https://github.com/milessabin/shapeless