Xamarin also supports the F# language
Xamarin has made a significant effort to also support F# in addition to C#.
FYI one of the designers of Swift previously worked on F# and you can see similarities between the two languages.
Why would one use F# over C#? There are many reasons but In general F# is more expressive and yields more concise, cleaner and safer code.
My first Xamarin / Android app was written in F# and ending up being about 6K lines-of-code. I have estimated that equivalent Java would be close to 50K loc.
Until Swift is ported to Android, F# is the only viable functional programming language for mobile development.