The Intuition project has had support for fallback languages since the beginning.
It has current language -> (if available: fallback lanuage ->) English.
However the fallback system is not as complete as MediaWiki's. Currently there are 13 fallbacks defined.
They can be found here
Sounds like you might want to write a little PHP script to extract the fallback configuration from MediaWiki to make this work. Shouldn't be too hard... Grep "$fallback" on phase3/languages/messages/Messages*.php.