As part of bug 1060825, we realised there's no en-US fallback, we need to consider what should happen if there's missing strings for locales, and ensure that is handled.
I suspect that if we're missing a sufficiently small number of strings, we might prefer to fall back only the missing strings, and use all the ones we've got. In some ideal world, that is. :-)
So actually, bug 1062835 totally fixes this.