It's actually chisel. But github marks it as scala because chisel is based on scala.
Other HDL based on scala includes SpinalHDL.
These two HDL are pretty welcomed because of opensource and usability. You can find lots of projects in them. Notably rocket-chip, which is also in chisel. https://github.com/chipsalliance/rocket-chip
They're around for a while but still young and some people are pushing for adoption.
It is scala. Chisel is just a scala framework. Conceptually it's not quite an HDL but instead a scala program that metaprograms the actual RTL netlist. So there's no chisel compiler other than the standard scala one that comes with sbt; you then run the resulting chisel program on your computer to generate the netlist.
Other HDL based on scala includes SpinalHDL.
These two HDL are pretty welcomed because of opensource and usability. You can find lots of projects in them. Notably rocket-chip, which is also in chisel. https://github.com/chipsalliance/rocket-chip
They're around for a while but still young and some people are pushing for adoption.