Documentation for multilingual site with shared files

Forwarded from : http://drupal.org/node/254214
"After browsing through drupal.org for more than three hours: Could someone please scribble together a how to with the proper setup of a multilingual site with subdomains (one per language) and shared files?

Obviously it neither works to make a multisite setup (sites/en.domain.tld, sites/fr.domain.tld ...), and
$conf['i18n_variables'] = array(
'file_directory_path',
);
seems not be work, either :-(

Thank you!"