alt Hacker NewsTIL - YouTrack was first released in 2009, Kotlin in 2011, so I wouldn't call it long before, but definitely before. Guess I'm more interested in programming languages than in task management tools, that's probably why I have been aware of Kotlin longer than of YouTrack...
Kotlin 1.0 was released early 2016. I can see why YouTrack team might not have wanted to commit using an unstable language, though I can also see the dogfooding angle.