It's implemented in Lua.
"40K lines of code have been rewritten (some manually, some with the help of a custom transpiler tool developed just for the task) from Scheme to l2l, a Lua-based language similar to Lisp. This allows the very sizable scripting side of Galactology to be based on LuaJIT, for the maximum possible performance for this kind of programming languages. sol2 provides the C++ binding layer for Lua.
The performance improvement is roughly 4x to 6x. In addition GC pauses are dramatically shorter thanks to Lua(JIT) incremental collector."