I don't agree when you say it would have been better if she'd been French... She's part of the world that Mika created, so he brings the world he created to us when he does a gig... And in an extreme way, since they are not a band, but Mika singer and musicians hired, he could have changed the musicians for them to speak the language of the country he's going to... But this is extreme.
I think she's part of the staff, like everyone there. And they can't change the staff as they change countries ! Way too complicated.