Pass preferred language in registration flow to Iterable

What does this MR do and why?

We want to pass preferred_language as a new/additional `property with what we already pass to Iterable at signup.

Related to #424946

How to set up and validate locally

1. Setup to simulate SaaS and restart GDK
2. Register new user.
3. On the welcome page select Just me and Join a project.
4. Submit the form. Go to /admin/sidekiq.
5. You can see the updated params in CreateIterableWorker failed job, sometimes you may need to restart GDK to see the failed jobs.
6. 

MR acceptance checklist

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