0003 — Internationalization

Status

Accepted.

Context

The product must support multiple languages. The repository also forbids hardcoded user-facing strings. A single mechanism should satisfy both.

Decision

• Use react-i18next in apps/web, configured in shared/i18n and provided at the app layer.
• Centralize all copy in locale resources under shared/i18n/locales/<lang>/<namespace>.json. Components reference keys only.
• Ship English (default/fallback) and Chinese. Language is detected from localStorage + browser and persisted; a language switch lives in the app shell.
• Namespaces mirror surfaces: common, trips, planner, auth.
• Numbers/currency go through shared Intl-based formatters; the trip currency (JPY) is data-driven, independent of UI language.

Consequences

• Adding a language is additive: drop in a locale folder, register it, add a switch label.
• The "no hardcoded strings" rule is satisfied by construction.
• Slightly more indirection when writing components (keys instead of literals), accepted for translation coverage and consistency. See ../frontend/i18n.md.