Dogs can hear. Why wouldn't their dreams contain what they hear in real life just like our dreams?
Because like we don't understand barks and most of them sound the same dogs don't understand our language except a set of commands we teach them
Ever had a barking dog in a dream, even though you don't understand barking? I think you don't realize that we can only dream what we've experienced, or put another way we specifically dream things we've experienced. Similar to how we can't imagine something we've never experienced.
Maybe the dogs can mix words in their dream, like when trying to remember a speech in a language that you don't know.