Currently, I have been working on a complex Drupal project with around 400 subdomains and half a million nodes. All the subdomains have same codebase. The product is in Chinese language and my real issue was to translate the strings from English to Chinese but the problem was that if a string is translated in one subdomain, it has to be translated to other 399 subdomains as well, so I decided to use the translation table of one (parent) domain on all other subdomains. So I just added the following lines to the settings.php of all 400 domains:
$databases['default']['default']['prefix'] = array(
'default' => 'mysite_',
'locales_target' => 'parent_db.mysite_',
'locales_source' => 'parent_db.mysite_',
“parent_db” is the database of the parent subdomain. Now if you change the translation of any subdomain, it will use the table of “parent_db” and will be changed in all 400 subdomains.