I recommend Go hands down. It's fast, cross-platform out of the box, easy to set up even on Windows, has the lowest learning curve (practically none to those who know C, C++, C#, Java, basically any other C-like language), secure by default, has a great standard library, and was made with microservices in mind.
Python is interpreted and can't cut the performance Go can, and being ducktyped, Python is prone to runtime errors that are hard to track down.
.NET requires a large, proprietary framework.
Java also requires a framework to be installed, and building it is not very intuitive; moreover, many people who are not Java devs are unwilling to go through the effort of setting up a Java workspace just to contribute.
NodeJS may offer a quick turnaround, but it has its own caveats, including a bloat-filled npm system, and marginal performance -- I've worked on several projects where we moved from Python to Node, expecting a speedup, but getting substantially worse CPU usage and increased memory usage as well.
I don't have much experience with Lua, but again, being interpreted, it's not ideal for a service. Erlang sounds good, but it's not as widespread as Go, and I don't believe it's as intuitive or easy to pick up, especially if we're looking for more contributors.