Nuxt/examples/other/locale/composables/locale.ts

import type { Ref } from 'vue'

export const useLocale = () => useState<string>('locale', () => useDefaultLocale().value)

export const useDefaultLocale = (fallback = 'en-US') => {
  const locale = ref(fallback)
  if (process.server) {
    // Learn more about the nuxtApp interface on https://v3.nuxtjs.org/docs/usage/nuxt-app#nuxtapp-interface-advanced
    const reqLocale = useRequestHeaders()['accept-language']?.split(',')[0]
    if (reqLocale) {
      locale.value = reqLocale
    }
  } else if (process.client) {
    const navLang = navigator.language
    if (navLang) {
      locale.value = navLang
    }
  }
  return locale
}

export const useLocales = () => {
  const locale = useLocale()
  const locales = ref([
    'en-US',
    'en-GB',
    'ko-KR',
    'zh-CN',
    'ar-EG',
    'fa-IR',
    'ja-JP-u-ca-japanese'
  ])
  if (!locales.value.includes(locale.value)) {
    locales.value.unshift(locale.value)
  }
  return locales
}

// Using Intl.DateTimeFormat for language-sensitive date and time formatting
// Learn more: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat
export const useLocaleDate = (date: Ref<Date> | Date, locale = useLocale()) => {
  return computed(() => new Intl.DateTimeFormat(locale.value, { dateStyle: 'full' }).format(unref(date)))
}