Limi has posted that this can be accomplished via workflow, but is somewhat short on hard details - except that PloneSolutions has done it for clients. Here is his most complete posting on the issue:

Essentially, we do this with workflow. We don't do real versioning, we use  
the History feature of ATCT if you're interested in the evolution of the  
document, and have the following workflow:

1. Canonical (source) document is added (in, for example, English)

2. Document is translated into German

3. Both documents are published

4. After a while, the Canonical document is updated. This sends a mail to  
the authors of the translations with the differences between the old and  
the new document, and invalidates the translations.

5. The translators update their documents, and changes the state back to  
"translated".