Michael from Campanella.se posts this issue with Aperture and accented letters in Swedish.
I have a similar problem with French accented letters. Say that I tag a friend’s name (“Étienne” was one in particular) in Faces. I can not build a smart album with all “Étienne” I had to rename him “Etienne” for it to work.
I do not know if it is because of accented capital letters or not. It could be since we Québécois do mark capital letters with accents while French (from France) do not.
I will try and will get back to you.