Kotlin is a promising language, but still pretty new, so it doesn't have as many libraries out there tailored for it yet or questions asked about weird corner cases you might encounter (so it can make it a bit harder to track down bugs). That has its positives if you want to be a pioneer in the language and start popping out some useful open source libraries though.
I love what Jetbrains does though and actively read their blog as they keep improving it. Kotlin has a bright future (I think some of the developers of Groovy are working with Jetbrains on it), but still a language in its infancy.
Scala may be a safer solution because Kotlin is newer, but that doesn't stop Kotlin being a better choice than other older languages like Groovy.
Today the Groovy P.M. announced they're adding statically-typed traits (http://groovy.329449.n5.nabble.com/Adding-Trait-to-Groovy-td...) to Groovy, talking about them like they're an innovation. But they were already implemented by former Groovy++ developer Alex Tkachman (https://code.google.com/p/groovypptest/wiki/Traits) 2 years ago. Groovy added static compilation into Groovy 1 year ago by copying it from Tkachman's Groovy++ codebase, then announced it to the world as "Groovy 2.0" without crediting him. Now they're trying the same trick again, laundering the Groovy++ Traits and passing it all off as "Groovy 2.2".
Alex Tkachman is now one of the Kotlin developers (along with Groovy's creator James Strachan). So which codebase would you trust: one laundered without credit from someone else's implementation (Groovy), or a second try from the actual person who wrote the first attempt (Kotlin)?
I love what Jetbrains does though and actively read their blog as they keep improving it. Kotlin has a bright future (I think some of the developers of Groovy are working with Jetbrains on it), but still a language in its infancy.