react-native-i18n.js

import React, { Component } from 'react'
import { Platform, Text, View, NativeModules } from 'react-native'
import styles from './styles'
import I18n from 'i18n-js'

function getLanguageByDevice() {
  return Platform.OS === 'ios'
    ? NativeModules.SettingsManager.settings.AppleLocale
    : NativeModules.I18nManager.localeIdentifier
}

const normalizeTranslate = {
  'en_US': 'en_US',
  'pt_BR': 'pt_BR',
  'en': 'en_US',
  'pt_US': 'pt_BR',
}

const en = {
  hello: 'Hello World',
}

const pt = {
  hello: 'Olá Mundo',
}

I18n.translations = {
  'en_US': en,
  'pt_BR': pt,
}

function setLanguageToI18n() {
  const language = getLanguageByDevice()
  const translateNormalize = normalizeTranslate[language]
  const iHaveThisLanguage = I18n.translations.hasOwnProperty(translateNormalize)
  iHaveThisLanguage
    ? I18n.locale = translateNormalize
    : I18n.defaultLocale = 'en_US'
}

setLanguageToI18n()

const translate = key => I18n.t(key)

class App extends Component {
  render() {
    return (
      <View style={styles.container}>
        <Text>i18N</Text>
        <Text>{translate('hello')}</Text>
      </View>
    )
  }
}

export default App