Internationalization & Localization Principles

i18n bugs are latent — they survive review unnoticed and only surface when the product expands to a new locale.

Locale-Aware Formatting

Use platform APIs that accept a locale tag; never hand-roll date or number rendering.

• Use Intl.DateTimeFormat(locale, options) and Intl.NumberFormat(locale, options) — not Date.toString(), toLocaleDateString() without a locale argument, or manual template strings.
• Pass BCP 47 locale tags (en-US, fr-FR, ar-EG); rely on CLDR data in the runtime rather than custom format maps.
• Never hard-code MM/DD/YYYY — format order, separator, and calendar system (Gregorian vs. Hijri vs. Buddhist Era) vary by locale.
• Currency: let Intl.NumberFormat(locale, { style: 'currency', currency: 'USD' }) place the symbol and decide sign position — never concatenate "$" + amount.
• For backend rendering, thread the Accept-Language header or a persisted locale preference down to the formatting layer.

Time Zones: Persist UTC, Render Local

Timestamps recorded in local time become ambiguous at DST boundaries and when users cross time zones.