Reproduce bug

1. Update applicationId & namespace in app/build.gradle to your app in firebase project.
2. Update app/google-services.json to your firebase project.
3. Run npm i in backend/functions directory followed by npm run deploy to deploy the cloud functions to your firebase project.
4. Run the app on an emulator and press the button in the bottom right hand corner to set the language, create a user and trigger the cloud function.
5. Head to the cloud function logs and observe the locale as authBeforeUserCreated {"locale":"und",....other properties}. "und" means undetermined locale. This should be "en-US".
6. In logcat, I observed the following log which explains why there is a bug:

2024-04-03 09:50:57.889 26056-26147 System                  io.flutter.plugins.firebase.tests    W  Ignoring header X-Firebase-Locale because its value was null.