Registrants should have a language

Problem/Motivation

Currently the registrant entity doesn't have a language associated with it. We should add one so we can capture the language used by the registrant at the time they submit the registration form. This is especially useful in multilingual websites. It will also allow us to make sure the notification mails are sent in the user's language, and event managers have a record of the language used by the registrant.

Proposed resolution

We can just set a langcode base field on the entity and let ContentEntityBase handle the heavy lifting. We should make sure to populate the language when a registrant entity is created.