Kotlin First means Kotlin instead of Java. Google wants to break apart from Java as Oracle is a legal troll and Java is a kind of legal Trojan Horse inside Android. Kotlin was created because Java is unproductive and missed desired features. Kotlin mixes functional programming and object orientation. Kotlin is concise, Java is verbose. That's all. Differently from Apple, Google won't harm any initiatives with other languages. If you code parts of your system in Java, please, "upgrade it" to Kotlin as soon as possible (few YEARS from now, you are plenty of time). Java is second class citizen now, just it. Kotlin is JVM compatible, today. Not sure in the future.
The only thing Xojo must have in mind is that they are entering in the ecosystem too late, and in few years from now will need to think again in terms of "Fuchsia", that, by the way, already have compilers for Rust, Kotlin, Swift, CLang, Dart... But Android still have a long run, go for it.