Matrix refers to a specification, not an implementation. There is nothing stopping you, or anyone, from implementing a Matrix server in whatever language you'd like. In fact, there is already a server being implemented in Go called Dendrite,[1] another called Ruma[2] that is being implemented in Rust, a "WIP toy" Elixir implementation called Matrex,[3] as well as others.[4]

[1] https://github.com/matrix-org/dendrite

[2] https://github.com/ruma/ruma

[3] https://github.com/bismark/matrex

[4] https://matrix.org/docs/projects/try-matrix-now.html#servers