I've never heard of language lock in. The source is here: https://github.com/JetBrains/kotlin

Come in; the water is fine.

There are many systems written in Cobol, Delphi, C++, that would really benefit from not being written in those languages.

There are serious headaches to be had when you have to maintain software for ten years and up.

Porting systems to another language is often terribly expensive.