Map<Basic, Embeddable> does not work, when the map key is a persistent field of the value. The map key shall not be represented as a separate table column, this is why the example uses @MapKey and not @MapKeyColumn.
Hibernate appears to expect an Entity as map value:
org.hibernate.AnnotationException: Associated class not found: LocalizedString
Detailed discussion of this use case:
Hibernate 3.5.3, PostgreSQL 8.4.3