i thought the moombas all say "laguna" because that's the only english word they've learned how to say.
in shumi village, a shumi explains that laguna tried to teach moombas how to speak, and the elder said that they didn't need words to communicate.
i thought that was why the moombas only say laguna. because laguna tried to teach them how to speak, but didn't do so well, and only got them to learn his name. i'm sure that's just where he started too.