If you know C# you don't really need Kotlin. You can write windows, windows universal, web, linux console, Android and iOS apps, all native, and all in C#.
I don't think that's necessarily true. Dotnet is still just getting started on Linux, meanwhile the Java ecosystem is insanely mature on Linux. Fwiw, I have been writing dotnetcore lately, and enjoying it, but it definitely doesn't match the java ecosystem in terms of library maturity. Because of Google/Netflix/etc, Java on Linux is a really interesting beast
Not to mention that toying with another language is not such a complicated endeavor once you're 5-10 languages in. The great thing about C# and Kotlin is they've been good at adopting features from other languages that are already good and familiar, so picking them up is generally super easy.
The Mono runtime is quite mature and capable on Linux as the Java ecosystem. Many very good GTK/Gnome apps have been written in C# over the years (Banshee, F-Spot, etc).
.NET Core is awesome, but don't forget that it's not the first time .NET has been available to Linux developers.
Not to mention that toying with another language is not such a complicated endeavor once you're 5-10 languages in. The great thing about C# and Kotlin is they've been good at adopting features from other languages that are already good and familiar, so picking them up is generally super easy.