I think mutual intelligibility between individuals goes beyond just shared vocabulary or grammar. It encompasses shared references, similar thinking patterns, compatible communication styles, and even emotional resonance. Some people just "speak our language" in a deeper sense.

Deutscher's work is particularly interesting here because it positions language somewhere between the rigid universalist position of Chomsky and the extreme relativism of Whorf. We're shaped by our language, yet not imprisoned by it.

The question of why we connect more easily with certain people could be partially explained by overlapping idiolects—similar metaphorical frameworks, comparable abstracting tendencies, or even shared cultural touchpoints that enhance understanding. Perhaps measuring that overlap could quantify why some conversations flow effortlessly while others feel like translation work.