Kotlin
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Kotlin is used successfully in production by multiple teams.
Kotlin is 100% interoperable with Java. It means the code can live side-by-side in one code base and interact. From the beginning it was designed with practical thought in mind. So the IDE Support in IntelliJ is really great.
The Spring Framework Developer put a lot of effort that Springs play well together with Kotlin.
With its concise syntax, null safety, Due to its explicit type system, this language is also great replacement for Groovy usage with Gradle.