Akamaru can understand human talk as he knows what Kiba is saying and Kiba understands dog talk as he understands what Akamaru says. So maybe they never had to teach him how to actually speak.
I guess..Does make sense but i mean there should atleast be a background reason like he was traumatized or something because the other dogs in the series talk....
Maybe he was slowly starting to learn it and then he hit his head too hard against the Rashomon that Sakon/Ukon summoned and now he can only speak in dog.