2nd layer application design isn't greatly different from design of a regular application, so there's no special language recommendation for it. The best choice will generally depend on two factors: which languages your devs know well and what's the nature of your application. For many common applications, you'll want to choose a language that has many libraries that can provide pre-written functionality for your application. For small applications, a rapid-prototyping language like Python is a good choice. But for large and complex applications, with many developers, you may want to consider a strongly typed language such as C++, especially if computational performance is paramount. And as I mentioned in the post, the best solution may be to use multiple languages, where your application's functionality is divided across several micro services.