Software languages have pros and cons, and are usually chosen accordingly. In this context, it is common to involve different languages in the development of complex systems, each one specifically tailored for a given concern. However, these languages create de facto silos, and offer little support for interoperability with other languages, be it statically or at runtime. In this paper, we report on our experiment on extracting a relevant behavioral interface from an existing language, and using it to enable interoperability at runtime. In particular, we present a systematic approach to define the behavioral interface and we discuss the expertise required to define it. We illustrate our work on the case study of SciHook, a C++ library enabling the runtime instrumentation of scientific software in Python. We present how the proposed approach, combined with SciHook, enables interoperability between Python and a domain-specific language dedicated to numerical analysis, namely \NabLab, and discuss overhead at runtime.