fix: Ensure use correct format locales

react/providers/I18n/format.jsx

@@ -6,18 +6,38 @@ import { DEFAULT_LANG } from '.'
 const locales = {}
 let lang = DEFAULT_LANG
 
+/**
+ * Ensure that the locale is in the correct format for date-fns (see 2.0.0 BC https://github.com/date-fns/date-fns/blob/main/CHANGELOG.md#200---2019-08-20)
+ * @param {string} lang
+ */
+const ensureLocaleFormat = lang => {
+  switch (lang) {
+    case 'en':
+      return 'en-US'
+    case 'zh_cn':
+      return 'zh-CN'
+    case 'zh_tw':
+      return 'zh-TW'
+    default:
+      return lang
+  }
+}
+
 const getWarningMessage = lang =>
   `The "${lang}" locale isn't supported by date-fns. or has not been included in the build. Check if you have configured a ContextReplacementPlugin that is too restrictive.`
 
 export const provideDateFnsLocale = (userLang, defaultLang = DEFAULT_LANG) => {
-  lang = userLang
+  lang = ensureLocaleFormat(userLang)
+  const ensureDefaultLang = ensureLocaleFormat(defaultLang)
   try {
-    locales[defaultLang] = require(`date-fns/locale/${defaultLang}/index.js`)
+    locales[
+      ensureDefaultLang
+    ] = require(`date-fns/locale/${ensureDefaultLang}/index.js`)
   } catch (err) {
-    console.warn(getWarningMessage(defaultLang))
+    console.warn(getWarningMessage(ensureDefaultLang))
   }
 
-  if (lang && lang !== defaultLang) {
+  if (lang && lang !== ensureDefaultLang) {
     try {
       locales[lang] = require(`date-fns/locale/${lang}/index.js`)
     } catch (e) {

react/providers/I18n/format.spec.jsx

@@ -10,6 +10,18 @@ describe('initFormat', () => {
   it('should not throw if a date-fns locale can not be found', () => {
     expect(() => initFormat('unknown-lang', 'unknown-default')).not.toThrow()
   })
+
+  it('should use the correct locale', () => {
+    const f = initFormat('fr', 'en')
+    const date = f(0, 'yyyy-LLLL-dd')
+    expect(date).toBe('1970-janvier-01')
+  })
+
+  it('should fallback english if a date-fns locale can not be found', () => {
+    const f = initFormat('unknown-lang', 'unknown-default')
+    const date = f(0, 'yyyy-LLLL-dd')
+    expect(date).toBe('1970-January-01')
+  })
 })
 
 describe('formatLocallyDistanceToNow', () => {