locale-format-validator

locale-format-validator

Overview

Locale-specific formatting is a common bug source:

• US user sees 5/4/2026 and reads "May 4." Spanish user sees the same string and reads "April 5."
• German user sees 1,234.56 and may interpret as 1.23456 (the comma is a thousands separator, the period a decimal in DE).
• Japanese user sees a price in dollars instead of yen.

This skill verifies the app uses the right format per locale.

When to use

• The product displays dates, numbers, currencies, or times.
• A bug report says "the date is in the wrong format."
• Pre-release: scheduled locale-format check.

Step 1 - Identify the formatting library

Stack	Library
JS / browser	Intl.DateTimeFormat / Intl.NumberFormat (built-in)
React	react-intl (FormatJS) <FormattedDate>
Python	babel.dates / babel.numbers
Java	DateTimeFormatter / NumberFormat with Locale
Ruby on Rails	I18n.l(date, format: :long) / number_to_currency
.NET	CultureInfo + ToString("d", culture)

The Intl APIs (browser-native) are recommended - they delegate to the underlying ICU library for canonical CLDR-based formatting.

Step 2 - Date format expectations per locale

Locale	Short date	Medium	Long
en-US	5/4/2026	May 4, 2026	May 4, 2026
en-GB	04/05/2026	4 May 2026	4 May 2026
de-DE	04.05.2026	04.05.2026	4. Mai 2026
fr-FR	04/05/2026	4 mai 2026	4 mai 2026
ja-JP	2026/05/04	2026/05/04	2026年5月4日
zh-CN	2026/5/4	2026年5月4日	2026年5月4日

US and DE date formats look superficially similar (differ only in separator) but are unambiguously interpreted only in their locale.

Step 3 - Number format expectations

Locale	1234.56	1234567
en-US	1,234.56	1,234,567
en-GB	1,234.56	1,234,567
de-DE	1.234,56	1.234.567
fr-FR	1 234,56	1 234 567
ar-SA	١٬٢٣٤٫٥٦	١٬٢٣٤٬٥٦٧
hi-IN	1,234.56	12,34,567

The Indian grouping (lakh = 100,000 displayed as 1,00,000) is a common bug source for apps using only Western thousands grouping.

Step 4 - Currency expectations

Locale	$1234.56 USD	€1234.56 EUR	¥1234 JPY
en-US	$1,234.56	€1,234.56	¥1,234
de-DE	1.234,56 $	1.234,56 €	1.234 ¥
fr-FR	1 234,56 $	1 234,56 €	1 234 ¥
ja-JP	$1,234.56	€1,234.56	¥1,234

Notes:

• Euro symbol position varies (€-DE vs DE-€ in some renderings).
• JPY has no decimals.
• Currency symbol may precede or follow per locale.

Step 5 - Author tests

import { test, expect } from '@playwright/test';

test('date format per locale', async ({ page }) => {
  // en-US
  await page.goto('/orders/123?lng=en-US');
  await expect(page.getByTestId('order-date')).toHaveText('5/4/2026');

  // de-DE
  await page.goto('/orders/123?lng=de-DE');
  await expect(page.getByTestId('order-date')).toHaveText('04.05.2026');

  // ja-JP
  await page.goto('/orders/123?lng=ja-JP');
  await expect(page.getByTestId('order-date')).toHaveText('2026/05/04');
});

test('currency format per locale', async ({ page }) => {
  await page.goto('/orders/123?lng=de-DE');
  await expect(page.getByTestId('order-total')).toHaveText('1.234,56 €');

  await page.goto('/orders/123?lng=ja-JP&currency=JPY');
  await expect(page.getByTestId('order-total')).toHaveText('¥1,235');   // no decimals
});

test('number format per locale', async ({ page }) => {
  await page.goto('/dashboard?lng=hi-IN&users=1234567');
  await expect(page.getByTestId('user-count')).toHaveText('12,34,567');   // Indian grouping
});

Step 6 - Time + timezone

A separate concern: timestamps must show in the user's timezone:

test('order time displays in user timezone', async ({ page }) => {
  await page.goto('/orders/123?lng=en-US&tz=America/New_York');
  await expect(page.getByTestId('order-time')).toContainText('EST');

  await page.goto('/orders/123?lng=ja-JP&tz=Asia/Tokyo');
  await expect(page.getByTestId('order-time')).toContainText('JST');
});

Step 7 - Locale matrix in CI

- name: Locale-format tests
  run: |
    for locale in en-US en-GB de-DE fr-FR ja-JP zh-CN ar-SA hi-IN; do
      LOCALE=$locale npx playwright test e2e/locale-formats/
    done

Or via Playwright projects:

// playwright.config.ts
projects: [
  { name: 'en-US', use: { locale: 'en-US' } },
  { name: 'de-DE', use: { locale: 'de-DE' } },
  // ...
],

Step 8 - CLDR as source of truth

The Unicode Common Locale Data Repository (CLDR) is the canonical source for locale data. Modern Intl libraries delegate to ICU which delegates to CLDR.

For verification, consult the CLDR data at cldr.unicode.org/. The format conventions in this skill come from CLDR; the actual expected strings should be derived from CLDR per locale, not hand-coded.

Anti-patterns

Anti-pattern	Why it fails	Fix
Hand-coded format strings	Drift from canonical CLDR; locale assumptions break.	Use Intl APIs / Intl.DateTimeFormat.
Universal "1,234.56" thousands grouping	Wrong for de-DE, fr-FR, hi-IN, etc.	Per-locale via Intl.
US-only date format	"5/4/2026" ambiguous internationally.	ISO 8601 (2026-05-04) for storage; locale-specific for display.
JPY with decimals	JPY has no decimal subdivision.	Per-currency precision; Intl handles automatically.
Timezone-naive timestamps	"Order placed at 14:00" - but in whose timezone?	Always include TZ; convert to user's TZ for display (Step 6).

Limitations

• Bicultural users. A Hindi user with English UI prefers Indian grouping; user-preference UI overrides locale auto-detection.
• CLDR updates. New CLDR releases sometimes change formatting; pin a CLDR version for stable test expectations.
• Server-vs-client formatting. Inconsistent if both render formats with different libraries; pick one.
• Intl API gaps. Some niche locales / formats are underrepresented in browser Intl; libraries like FormatJS fill some gaps.

References

• W3C i18n at w3.org/International/.
• CLDR at cldr.unicode.org/.
• i18n-string-coverage, pseudo-localization-runner, rtl-rendering-tester - sibling skills for the broader l10n test surface.