Adyghe (ady) and Kabardian (kbd) are spoken by different groups of Circassians in Russia, Turkey, Israel and other countries. Russian is useful as a fallback language for Circassians in Russia, but it is useless for Circassians in other countries; It's more likely that they know English and not Russian.
I discussed this with User:Богупс (Bogups), the main editor of the Incubator of the Kabardian Wikipedia, and he agreed that it should be done, even though he doesn't know English so well himself. (See http://incubator.wikimedia.org/wiki/User_talk:%D0%91%D0%BE%D0%B3%D1%83%D0%BF%D1%81#Fallback_language ; it's in Russian.)
It is also possible to make ady and kbd each other's fallback languages, because they are closely related and at least partly mutually intelligible.
For the record, it would be much better to allow users select their own fallback languages, as Bug 11267 suggests.