English to any language as a background task with Firebase cloud functions.

index.js

var functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);

const request = require('request-promise');
const _ = require('lodash');

// List of output languages.
const LANGUAGES = ['es', 'fr', 'ar'];


exports.translate = functions.database.ref('/translations/{translationId}').onWrite(event => {
  const snapshot = event.data;
  const promises = [];


  _.each(LANGUAGES, (lang) => {
      console.log(lang)
      promises.push(createTranslationPromise(lang, snapshot));
   })

  return Promise.all(promises)

});


// URL to the Google Translate API.
function createTranslateUrl(lang, text) {
  return `https://www.googleapis.com/language/translate/v2?key=${functions.config().firebase.apiKey}&source=en&target=${lang}&q=${text}`;
}

function createTranslationPromise(lang, snapshot) {
  const key = snapshot.key;
  const text = snapshot.val().english;
  let translation = {}

  return request(createTranslateUrl(lang, text), {resolveWithFullResponse: true}).then(
      response => {
        if (response.statusCode === 200) {
          const resData = JSON.parse(response.body).data;


          translation[lang] = resData.translations[0].translatedText

          return admin.database().ref(`/translations/${key}`)
              .update(translation);
        }
        else throw response.body;
      });
}

package.json

{
  "name": "functions",
  "description": "Cloud Functions for Firebase",
  "dependencies": {
    "firebase-admin": "^4.1.2",
    "firebase-functions": "^0.5",
    "lodash": "^4.17.4",
    "request-promise": "^2.0.0",
  },
  "private": true
}