"She is a sweet girl."
Translation:È una ragazza dolce.
No, adjectives in Italian are usually placed after the noun, but there are special cases: http://italian.about.com/library/fare/blfare129a.htm
When an adjective can precede a noun it usually has a figurative meaning and when it can follow it, it usually has a literal meaning. I would assume that sweet in the sense of 'sugary' is literal and in the sense of 'endearingly good natured' is figurative. Comments, anyone?
I would guess the Italians use "al" kind of like we use "al" or "a la" in Spanish (my first language) to denote how something is cooked or prepared. For example: Papas a la francesa = french fries
So "ragazza al dolce" sounds to me like "sweetened girl". Anyway, I'm no expert, just a guess based on the similarities between Spanish and Italian.
because "al" indicates a flavor or the word with, for example "torta al cioccolato" is translated as a chocolate cake or a cake with chocolate, so in this case if you said "lei è una ragazza al dolce" it would be translated to she's a girl with a sweet taste, or a girl with sweet, which doesn't make that much sense :) hope i could help :)