i18n Formatting: Dates, Numbers & Currency with Intl and i18next (2026 Guide)

Hand-rolled “$” + toFixed(2) formatting is a bug in most locales: 1,234.56 is 1.234,56 in German and 1 234 567,89 in French, and yen has no decimal places. This guide covers Intl.NumberFormat, DateTimeFormat, RelativeTimeFormat, and how i18next wires them into translation strings with { {val, number} }, currency, datetime and custom formatters.