I'm guessing the downvotes here are based on your wording, "worst possible thing to do", and not your core point that Korean and Japanese are grammatically very

I'm guessing the downvotes here are based on your wording, "worst possible thing to do", and not your core point that Korean and Japanese are grammatically very similar. You're right that the test would be even more interesting with three very different languages. But it's still pretty impressive with Korean, English, and Japanese as shown, despite the similarity.