casual is the hardest register

Everyone assumes formal Japanese is the hard part, all that keigo. It’s backwards. Formal Japanese is rule-bound and learnable. Casual is where you sound native or you don’t, and there’s no rulebook for じゃん vs よ vs 草.

That’s the reason the tone translator exists. A literal translator gives you stiff textbook output that reads as non-native on sight. A chatbot can do better, but you re-explain “natural, casual, no romaji” every session and the context drifts as the chat grows. The product is one tuned, hardened prompt for naturalness behind a one-tap interface.

The general point: the impressive case usually isn’t the formal, structured one. It’s the messy “you just have to know” case, and that’s the one worth building for.