If you've seen the first Star Wars trilogy (meaning Episodes 4 - 6), in the original Star Wars movie, Han Solo is in the cantina talking with Greedo (and yes, darnit, Han shot first! no shame in that, Greedo posed a mortal danger to him). But each is speaking in his own language, Han in English (Empirish?) and Greedo in his native language. Yet the conversation proceeded normally -- for a little while, at least.
I have a personal experience with this phenomenon, having once been in the US Army in the Netherlands attending a NATO class on how to maintain a particular kind of microwave radio, and for the lab portion of the class I was paired with a German soldier. It happened that we were both bilingual in English and German, and so rather than both of us using one or the other language, I proposed that we speak our native languages, and see how that would work -- both of us understood each other's language better than we spoke it. So we did and it worked out really well. I imagine that it sounded darned funny to some of the other US soldiers in the room who overheard our conversation (the class was taught in English, so non-US and -Brit soldiers had to have English as an additional language to attend).
Designating one's native language as A, and a second language as B, is there a word for this kind of conversation where the two participants speak their own language A while understanding in their language B?