" Todo lo que brilla no es oro"
Perro que ladra no muerde.
His bark is worse than his bite (literally: a dog that barks doesn't bite)
Another expression for "All that glitters is not gold" is "No es oro todo lo que reluce"
Relucir = to twinkle - glitter
I love these expressions/sayings/idioms/proverbs. We have been doing them in class - however I do feel that you have to have a reasonable level of conversational Spanish to be able to use them!