casual is the hardest register

Everyone assumes formal language 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.

This is the whole reason 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 single time and the context drifts as the chat grows. The product is one tuned, hardened prompt for naturalness behind a one-tap interface — so casual comes out sounding like a person, every time.

The lesson generalises past Japanese: the impressive thing usually isn’t the formal, structured case. It’s the messy, unwritten, “you just have to know” case — and that’s the one worth building for.