Add preferred_language to lead generation for registration

What does this MR do and why?

Add preferred_language to lead generation for registration

• translate the language to a shortened format, see https://gitlab.com/gitlab-org/gitlab/-/issues/424946#note_1607384072
• needed for iterable and other lead concerns
• CustomersDot support in https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/8434

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #424946