What's the best way to get device locale in react native (iOS)?

Question

There's no need for an external library. You can find what you're looking for in React's Native Modules

import { NativeModules } from 'react-native'

// iOS:
const locale = NativeModules.SettingsManager.settings.AppleLocale ||
               NativeModules.SettingsManager.settings.AppleLanguages[0] // "fr_FR"

// Android:
const locale = NativeModules.I18nManager.localeIdentifier // "fr_FR"

To test this, I changed the language on my device to French. Here's a sample of what you'll find in the NativeModules.SettingsManager.settings object related to locale:

{
    ...
    AppleKeyboards: [
        "fr_FR@hw=US;sw=QWERTY",
        "en_US@sw=QWERTY;hw=Automatic",
        "es_419@sw=QWERTY-Spanish;hw=Automatic",
        "emoji@sw=Emoji"
    ]
    AppleLanguages: ["fr-US", "en-US", "es-US", "en"]
    AppleLanguagesDidMigrate: "9.2"
    AppleLocale: "fr_FR"
    NSLanguages: ["fr-US", "en-US", "es-US", "en"]
    ...
}

import { Platform, NativeModules } from 'react-native'

const deviceLanguage =
      Platform.OS === 'ios'
        ? NativeModules.SettingsManager.settings.AppleLocale ||
          NativeModules.SettingsManager.settings.AppleLanguages[0] //iOS 13
        : NativeModules.I18nManager.localeIdentifier;

console.log(deviceLanguage); //en_US

I am using the i18n package (react-native-i18n). And then it's just:

I18n = require('react-native-i18n')
locale = I18n.currentLocale()

Nothing of the above worked for me, but this component.

console.log("Device Locale", DeviceInfo.getDeviceLocale()); // e.g en-US

function getLocale() {
  if (React.Platform.OS === 'android') {
    return I18n.locale;
  } else {
    return NativeModules.SettingsManager.settings.AppleLocale.replace(/_/, '-');
  }
}

iOS 13 workaround here:

locale = NativeModules.SettingsManager.settings.AppleLocale // "fr_FR"
console.log(" ==> Current settings: ", NativeModules.SettingsManager.settings)
if (locale === undefined) {
    // iOS 13 workaround, take first of AppleLanguages array 
    locale = NativeModules.SettingsManager.settings.AppleLanguages[0]
    if (locale == undefined) {
          return "en" // default language
    }
}