The way beings in different realms communicate does not involve a language.
When we speak, in addition to the sound coming out, our thoughts “emit” the idea to the external world too. That idea can be grasped by those beings.
That is how we “hear” in dreams. While we dream we can hear those in the dream say. But we don’t hear anything through our ears.
I think you can get the basic idea by thinking about the following.
Consider someone who knows two (or more) languages well. When that person dreams, he does not hear with one of those languages. He just understands what someone in the dream says.
Another point is that Brahmas in Brahma realms do not have dense bodies like ours. They cannot “speak” as we do. They hear in the above way, just with their thoughts. They don’t have languages.
This was discussed in the post published today, with references to previous posts: “Mental Body Versus the Physical Body.”