I wonder if it would be a bit hand-wavy or improbable to suggest that maybe Cas and Hael weren't actually having that conversation in English. Maybe Hael didn't actually say "angel radio." I might be making intuitive leaps I shouldn't be making, but I would think that "angel radio" would be something fundamentally ingrained into angels and their culture and would probably have a way of referring to it in their own language. Humans, however, do not have a similar capability as part of their being and therefore wouldn't have a term for it. However, Dean has used the term "angel radio" before, and it might, perhaps be the closest way of understanding the term, so the writers chose to use the term because they knew their viewers would understand it instantly rather than come up with another term to describe the concept.