"Mangio frutta eccetto una."
It would be an acceptable translation if the Italian sentence said "all fruits". But it actually uses the singular 'frutta'.
It can actually only sound like real English if you say "all fruits" or "all fruit".
Out of context, it's not very natural in Italian as well... for you and HavardF.
I think it's supposed to have been read with a pause/comma. i.e. I eat fruit, except one.