do you speak English? = Parli inglese?/ Parla inglese?
the difference between the two sentences is that the first (parli inglese?) is usually referred to young people. It is an informal italian sentence. The second one (parla inglese?) is more formal and generally is referred to someone who is older than you and that you meet for the first time.
so if you meet me on the street you can say "parli inglese?".