I'm just finishing something of a large migration from an old Xoops site to Drupal. I've not had to get too down and dirty with the Xoops database, for which I'm quite happy as it wasn't so coherent after lots of upgrades and customisations.
One of the issues with user generated content multi-lingual sites is that content will often be in only one language, or maybe translated into a few others.
It's sometimes really cool to be able to share some of the tables between multiple Drupal sites. Just think your users have the same logins for example.