My suggestion would be to have one site altogether. That is translated into multiple languages (i.e Belgium, French, etc) and have multiple domains. When the domain for the French site is opened into the browser the sites language and currency reflects that of France and the same with Belgium.
You can then cloak the domain to stay as the .fr or .nl whilst the user visits the site. Magento Community Edition supports this with the use of a plugin or two.
