I'm wondering why you include both Kotlin and Java in the discussion of building microservice applications ?
I'm used to this with Android documentation and was wondering if we are now going the "switch to Kotlin" path with server side applications .
Spring is to large degree language-agnostic when it comes to Java vs. Kotlin. The goal is 100% parity between the two languages, and it's very close. Practically speaking, all Spring components are developed in Java, and Spring APIs for Kotlin are often more elegant owing to Kotlin's DSL capabilities...so it's a bit of give and take.
I don't think Java will be supplanted fully by Kotlin, ever. But it's a great thing to have options, and Spring Boot and other Spring libraries are easily consumed and leveraged from both, which is nice for shops which don't embrace both but strongly prefer one over the other.
There really isn't much to it to use both in the same app...both compile to the same bytecode and Kotlin coexists really nicely with Java code, even within the same project. For fun sometime, take a small sample Spring Boot project and use the Spring Initializr (at start.spring.io) to create a project with the same dependencies, but selecting Kotlin as the language. Then preview the pom.xml / gradle.build.kts, copy the Kotlin-specific material from that and paste into your sample project, and rebuild it. No changes, still Java code, but the project can now compile Kotlin sources.
Then convert a (or multiple) Java domain class(es) to Kotlin and rebuild the project. You'll likely need to adapt a few calls to accommodate the change in constructor(s) (assuming you fully embrace idiomatic Kotlin), but otherwise, everything works brilliantly.
So it's possible, and not even problematic in most cases. But you're right, not something you're likely to see very often.
Caveat: I really wasn't originally referring to shops that use Java and Kotlin in the same app, but rather those shops that may allow development teams to choose their language per project and thus end up with both, depending upon the team/application. The previous answer was just to demonstrate that mixing both languages together in the same project really isn't that big of a deal, and not to necessarily encourage that become the norm.