Create two wikis and use interwiki links between the two of them.
I think this would be quite possible, though slightly tedious. If there's no alternative, I may have to do this, or use TikiWiki... I'm going on a limb here, but I would think that the management of one tikiwiki would be easier than two mediawikis...
Unfortunately, it doesn't seem easy to modify the list of recognized interwiki links in MediaWiki. I know in TikiWiki there was an admin page for this, but MediaWiki doesn't have one.
I've seen some work on interwiki links here... http://meta.wikimedia.org/wiki/Interwiki_links though it seems poorly explained.
It also briefly mentions interlanguage links.
Are languages handled through separate installs of mediawiki and then interwiki linked, or are the languages defined within the same wiki? Basically, could I define my own "language", and then allow only english pages to be viewed by all users?