There was an interesting use case for the early zero language code. It allowed the checksum to remain intact when switching from early SBN to ISBN, all you had to do was prefix with zero. It isn't quite so simple today, if you find an old British book you can't simply tack a zero on the front.

It is mentioned in the Wikipedia link, but buried a little. I realise ISBN history is the epitome of super-nerdy, but the evolution path from $just_some_retailer through to the Bookland "country" is really quite interesting from an interoperability perspective.