Hi,
same problem within my multisite instalation.
I've setup custom domains within MU Domain Mapping and uncheck option "Disable primary domain check." because I don't want something like site1.wordpressmu.work like a primary domain. I need only newly created domains.
So I created site1.work for my default language and set as primary and site1en.work for second language. Second language domain is redirected to the first default language domain.
Sorry for my english :)