Localise notifications sent from Firebase

Is there a way to translate / localise notifications sent from Firebase?

I have my app setup to receive notifications successfully:

extension AppDelegate: UNUserNotificationCenterDelegate {

    func setupNotifications() {
        registerForRemoteNotifications()
        setupNotificationTokenRefresh()
    }

    func registerForRemoteNotifications() {
        let application = UIApplication.shared
        UNUserNotificationCenter.current().delegate = self

        application.registerForRemoteNotifications()
    }

    func setupNotificationTokenRefresh() {
        storeNotificationToken()
    
        _ = NotificationCenter.default.addObserver(
            forName: .MessagingRegistrationTokenRefreshed,
            object: nil,
            queue: .current
        ) { [weak self] _ in
            self?.storeNotificationToken()
        }
    }

    private func storeNotificationToken() {
        Messaging.messaging().token { [weak self] token, error in
            if let error = error {
                Log.error("Error fetching FCM registration token: \(error)")
            } else if let token = token {
                // save token
        }
    }
}

}

A payload is sent from a Firebase cloud function with a data object and I would like to access this data object and translate/localize the message sent.

I looked into several methods provided, but they seem to be about intercepting notifications only when the app is in the foreground, which is not what I am interested in.

The payload sent from the server:

const payload = {
    notification: {
        title: 'Friend request',
        body: senderName + ' wants to add you as a friend'
    },
    data: {
        senderUserId: friendRequestFrom,
        type: 'friendRequest'
    }
}

Because you are already using cloud functions, one way to accomplish that is to do the translation server-side with the Google Cloud Translation API. There is a good sample demonstrating how to do so with Node.js.

Say for example you are sending the notifications when new objects get added to the /notifications path in your real-time database. You can do something like this:

const Translate = require('@google-cloud/translate')
const functions = require('firebase-functions')
const projectId = 'YOUR_PROJECT_ID'
const translate = new Translate({
  projectId: projectId,
});
const admin = require('firebase-admin')
admin.initializeApp(functions.config().firebase)

exports.sendNotification = functions.database.ref(`/notifications/{notif_id}`)
  .onWrite(event => {
    const notification = event.data.val()
    // Don't send when this isn't a new notification.
    if (event.data.previous.exists()) {
      return null
    }
    const user_id = notification.user_id
    getLocalLanguageOfUser(user_id).then(language => {
      if (language != 'en')
        translateTo(language, notification).then(localised => {
          return send(localised)
        }).catch(err => {
          return console.log(err)
        })
      } else { // it's English - no need to translate
        return send(notification)
      }
    })
})

function getLocalLanguageOfUser(user_id) {
  return new Promise((resolve, reject) => {
    // default will be 'en' for English
    firebase.database().ref(`users/${user_id}/language`)
      .once('value').then(snapshot => {
        resolve(snapshot.val() || 'en')
      })
      .catch(err => reject(err))
  })
}

function translateTo(language, notification) {
  return new Promise((resolve, reject) => {
    const text = notification.text;
    translate.translate(text, language).then(results => {
      const translation = results[0];
      resolve({
        ...notification,
        text: translation
      })
    }).catch(err => reject(err))
  })
}

function send(notification) {
  // use your current setup to send the notification.
  // 'text' key will already be localised.
}