Large software systems often comprise programs written in different programming languages. In the case when cross-language interoperability is accomplished with a Foreign Function Interface (FFI), for example pybind11, Boost.Python, Emscripten, PyV8, or JNI, among many others, common software engineering tools, such as call-graph analysis, are obstructed by the opacity of the FFI. This complicates debugging and fosters potential inefficiency and security problems. One contributing issue is that there is little rigorous software design advice for multilingual software. In this paper, we present our progress towards a more rigorous design approach to multilingual software. The approach is based on the existing approach to the design of large-scale C++ systems developed by Lakos. The Lakosian approach is one of the few design methodologies to address physical design rather than just logical design. Using the MLSA toolkit developed in prior work for analysis of multilingual software, we focus in on one FFI -- the pybind11 FFI. An extension to the Lakosian C++ design rules is proposed to address multilingual software that uses pybind11. Using a sample of 50 public GitHub repositories that use pybind11, we measure how many repositories would currently satisfy these rules. We conclude with a proposed generalization of the pybind11-based rules for any multilingual software using an FFI interface.