Non-English editions do not get a slug

Because we cannot reliably generate readable slug strings for
non-English editions, we don't generate one, meaning that document
identification/param-generation will fallback to the document id.
Simplfy update slug logic

The unless / or clause was confusing so I've simplified it with a
guard clause. This uncovered a potential bug where the published?
check would return true even when the documents edition had been
unpublished because the published_edition association was returning
a cached edition.
Disable translations for non-English editions

If a user creates a non-English edition, then they forfeit their right
to translate it. We *could* in-theory allow this, but because the
specific use-case for this story is for news articles that are only
relevant to the local populous, we disable it to keep things simple.
Allow non-English world location news articles