@Jifeng, thank you very much for the answers. I think I get the picture now, great work!

However, one thing which bugs me is how to efficiently share a schema for the different databases. It's easy enough to set the up once, but making changes later seems to be fragile process because I have to make the modifications to all databases of all localized sites. It would be great if we had a separation of schema and data for the databases, so that you could have multiple databases sharing the same schema (especially in the light of content broadcasting).