It has to be said that there is a publication date of
September this year listed on Amazon.co.uk; never a fool-proof indicator of a book coming out, but at least a preliminary idea of when it
might be (they also show the cover).
I also wouldn't be surprised if the processes for abridgement and translation haven't been running in parallel with the general production. I've not been under the impression that new material has been being written for these books, rather that they are working from the main texts; it's therefore possible that the work on volume six was being done even while waiting for volume seven and even then it might have been possible to pass along portions of that to be translated as they were completed.