Android apps have been traditionally built using Java since the inception of Android. However, Google announced Kotlin as an official supported language for the Android platform in May 2017. Since then, the popularity of Kotlin for Android projects has steadily increased, to the point that Google announced in 2019 that “Android development will be Kotlin-first” with nearly 60% of the top 1,000 Android apps containing Kotlin code. Yet, the transition from Java to Kotlin seems gradual and most applications still partially use Java. Outside open-source apps, little is known about the real proportion of code written in Kotlin inside apps. This paper supports a better understanding of the adoption of Kotlin in the Android ecosystem. We propose an approach to identify the language, Java or Kotlin, in which a class bytecode of an Android Package Kit (APK) originate from. We applied our model on more than 200k closed-source APKs from app stores and found that (i) most of the apps classes are still written in Java, indicating a mitigated adoption of Kotlin in less popular apps, (ii) the penetration of Kotlin is steadily increasing since 2017. We believe our insights are valuable to assess the adoption of Kotlin at large.