Will an Amazon review appear in all locales?
As you're probably aware, Amazon has different websites for different geographic regions (ex. USA, United Kingdom, Australia, India, Mexico, etc...) also known as locales.
An Amazon reviewer can only write reviews for things in their locale.
So for example, a reviewer in the UK will only be able to review your book on Amazon UK.
When someone views your book on Amazon UK, they will see the review.
When someone views your book outside the UK, Amazon may or may not display the UK review under a section called Top reviews from other countries. To the best of our knowledge, the reviewer has no control over whether their review will be displayed under this "international" section.
At this time, it is not possible to target readers on BookSirens based on their Amazon locale. But it's a feature we hope to add in the future.