My Biggest Mistake: Why You Should i18n Your Next.js App From Day One (A Vibe Coding Survival Guide)

My Biggest Mistake: Why You Should i18n Your Next.js App From Day One I built a split-bill application called FAMI-KAN (originally for the Japanese market) using Next.js 15 App Router and Supabase. Since I built it mostly through "Vibe Coding" (coding iteratively with AI agents like Claude/Gemini), feature development was incredibly fast. I have absolutely zero professional programming experience, yet AI allowed me to build a fully functional product. I thought I was a genius. Then, X (Twitter) expanded its multilingual features. Suddenly, global traffic was a real possibility. I realized: If people from around the world click my app link from X and see only Japanese, my user journey is completely broken. So, I decided to translate my app into 8 languages. "It should be easy with AI, right?" I thought. I was dead wrong. Not building with i18n (next-intl) from Day 1 was the biggest mistake of this project. Even with the power of Vibe Coding, migrating an existing, monolithic codebase wa