Kotlin is not meant to be used only for Android development. There is a lot of backend code out there, all written in Java, and nothing should stop you adding Kotlin into the mix whenever you have to add new functionality. Don't get locked into Java as your only option when it comes to deciding on the JVM language to be used in your new project. When your new microservice-oriented system gets the green light for you to start coding, why not actually rely on Kotlin?
This chapter is not meant to be a deep dive into the realm of designing microservices, but rather a brush up on the terminology. There is a lot of documentation written on the topic of microservices, and you might have already been exposed to the principles; however, I encourage you to read Reactive Microservices Architecture: Design Principles for Distributed Systems, Jonas Bonér...