You can start by exploring previous posts related to Kotlin in the forum. I found two posts that are relevant:
My suggestion to prepare to GSoC? Become familiar with the PDE, learn how the PDE manages modes, how does java and android mode works and then explore what is needed to support Kotlin. Get involved here with the community by helping users and learning current use cases. Finally, this wiki doc might come handy to get started with Processing android.
Kf