Hi! Sorry if it's a bit late, I've just seen this. I did the same thing in England for boutiques etc, in March. If you said you speak other languages you will do most of the interview in french (or english) then switch to the other languages just for a chat (along the lines of 'have you ever visited spain?'), it's pretty simple stuff. (:
I got asked about why i want to work there, if I'd ever visited and what I enjoyed there, plus whereabouts you'd wanna work.
I wore a dress and little jacket to my interview, but you can really wear whatever you want, just no jeans obviously! A blazer would be perfect I think (:
GOOD LUCK! When do you want to start?