The lang module/system/library can be loaded from multiple programming languages to seamlessly integrate all the other supported ones. The currently supported languages are Python and Lisp.
Things I (or some contributor?) plan to do next, in no particular order. Some of these ideas can probably be implemented in a few hours and others will be near impossible, but it is hard to tell which are which in advance.
We can already convert Python modules into Lisp packages, and all the necessary machinery is there. The next step is to turn Lisp packages into Python modules. The plan is to fill each resulting Python module according to some fuzzy heuristics, and give each module foo two submodules foo.__lisp_functions__ and foo.__lisp_values__ that accurately reflect the two Lisp namespaces. Unless there is a collision, these submodules have the aliases foo.functions and foo.values. When a Lisp package name contains dots, we turn it into several nested modules.
A possible next step would be to not just convert packages to modules, but to turn them into modules directly, i.e., to have Lisp packages inherit from the Python module type.