Need for an introduction to internationalization features

It seems that something has been a bit forgotten. Before starting, users have to choose a CMS, among very many. So we need precise information about the features of CMSs but without the technical way to use them.
An important point for me is internationalization. What does yours do exactly? I imagine that all contributors of a one project can have interfaces in their own languages. Am I right?
But what happens if an article doesn't exist in the reader's language but exists in another language he/she can understand? Many people can read more than one language and would prefer an article in one of them than no article at all. On the other hand, an article in a language they can't undersatnad is useless to them. Now, international web sites whose all articles exist in all their languages are rare. Are one user's several languages hierarchy easily managed in Drupal?
Is the notion of "original language" (or reference language) of an article easily managed? It's important in case of doubt on a term or the meaning of a sentence, in international Law or History for instance.