Just to add another fact to the discussion, I've just come across a reference to a study Transport Canada did, investigating the safety issues surrounding bilingual ATC. This study took place between 1976 and 1981 (sorry, can't find the report, but could probably get it from the people involved or the TC library).
Transport Canada regs still allow bilingual ATC, so I guess they decided there was no significant danger (your starter for ten: define 'significant'...).
