I agree that I would not by my own choice mix many languages in one project. I also agree that homogenizing naming conventions (so that the language transitions will be next to invisible) is probably not going to happen. Many language communities did not even achieve this kind of homogeneity in their own language ecosystem (e.g. C++).
Yet I strongly disagree that this is not a proper and important use case. Some special purpose libraries are just not written in your favorite language. Being able to integrate them easily is fantastic.