As I understand it, C++ to maintain the QPX in-memory database and SBCL to do the routing magic (which is even more non-trivial than you might think, e.g. a "best" result set is not 20 routes that are all within a dollar and that hit the same airports, etc. etc.).
RES uses Clozure CL (formerly OpenMCL, an evolution of Macintosh Common Lisp) for the stateless middleware with Oracle RAC for the backend and Java with the usual libraries for the web front end.