Being curious about the implementation language. Viewing the code is not easy.
If you guess Fortran, you might be right:
(different ICON Project) "The infrastructure, ICON-Land, for this ICON-A land component has been newly designed in a Fortran2008 object-oriented, modular, and flexible way."
https://mpimet.mpg.de/fileadmin/publikationen/Reports/WEB_Bz...
Fortran alive and kicking: