Coordinate Reference System Transformations

"JVM library" implemented with Kotlin, but usable from Java.
Most of the test code is written with Java, and the third-part libraries used from Kotlin are also implemented with Java.
github TomasJohansson/crsTransformations

.NET library implemented with F#, but also usable from C#.
Most of the test code is actually written with C#. The third-part libraries used from the F# code may also have been implemented with C#.
github TomasJohansson/crsTransformations-dotnet

Wikipedia Geographic coordinate conversion

Tomas Johansson at GitHub