Internationalization (i18n)

Overview

Designing software to adapt to languages/regions without code changes (i18n), then adapting it per-locale (l10n). Covers translation architecture, ICU pluralization, Intl-based formatting, RTL/bidi, and libraries (i18next, react-intl/FormatJS, gettext).

The Rules That Prevent Rework

1. Never concatenate translated fragments. Word order, gender agreement, and grammar differ per language. Use one full-sentence key with named placeholders; the translator controls order.

   // Wrong — impossible to translate; order is baked into code
   t("You have") + " " + count + " " + t("new messages");
   // Right — one message, interpolation + plural inside it
   t("inbox.newMessages", { count }); // "{count, plural, one {# new message} other {# new messages}}"