Why use something that works 90 times out 100, but blows up ten times, versus an automated process that can be provably correct all the time?

This is an application where LLMs should be the obvious choice, it is Machine translation the thing they are supposed to excel at. Why not use them? Likely a lack of data. You would need lots of Kotlin data (I’m sure lots of Java data exists), and the data would need to overlap so the LLM could understand the mapping.