Bonus question: what do you do if data needs to stay in the creating user's region by default, but users can share data across regions, including creating new data associated with a record from another region?

For example, American and EU users sharing documents and requesting comments be added. How do you manage both isolation and sharing? Do you copy such documents to both regions? Do comments live in the same region as the document, or in the region of the creating user? Do you just give up on foreign key integrity for documents, comments, and users because they could be in separate databases? If your American servers can request data out of the European region on demand (subject to business logic), are you really protecting data to the standard the law demands?