When people commonly speak of fetishes, they're referring to what's known in psychiatry as "fetishism." One example is the underwear fetish. At what point is this considered an illness?!
You don't know much? 
I'd say you know too much.
Du kennst dich nicht aus?
Ich glaube, du kennst dich zu gut aus.
Para decir que no estás "muy segura", 
tienes suficientes conocimientos.
Para decir que no sabes mucho, 
tienes suficientes conocimientos.
Loin d’être ignorante,
vous en savez un peu trop.
Non ne sai molto? 
Direi che sai anche troppo.
E não sabe muito? Eu diria que sabe muito!