[MyModule.Employee_region = $CurrentAccount/Account_Region]
You can use something like that. You need to adjust it based on the naming of your associations, modules and entities.
If you want to do it on a page and not in a microflow, you may need to have a dataview showing the current user so that you can use it as “currentObject”
If your user is always only allowed to see this set of Employees, you can also add an xpath to the access rules of your employee entity and only give read permissions on those employees that are associated to a region that is associated with the user.