...Yeah, now that I think about it. The Watatsuki sisters have some justification for being able to speak Japanese (called upon shinto god, for once), but no way regular lunar rabbit would instantly know japanese if the elitist like Lunarian have their own language to speak of, since that would most likely be what they learn, unless 1. The Lunanese is speak in Japanese and have the exact same structural and tone, 2. Lunarian's education is miles and miles ahead of our or 3. It's still actually Japanese, but the literal Moonrunes Kaguya have is an encryption, to prevent anyone except them to read it.