Sprechen Sie Englisch vs Spechen Sie Englisch
ones a polite order and another is asking if they can speak English.
Is it just the way you pronounce these sentences to be able to get you point across?
this order is surely far from being polite. if you want to make it seem polite you have add a bitte for please.:
"Bitte sprechen Sie Englisch" or "Sprechen Sie bitte Englisch"
actually you shouldn't order people to speak english especially in a country that is not an english speaking country. in this case an order to speak english will always come as rude. it is always better to ask for it like: "Könnten Sie bitte Englisch sprechen?" this of course will only work if the other person knows english. nowadays there are many people in german speaking countries who speak english but there are still a lot of people who can't.
as for the question of pronunciation. the words are pronunced the same way in both cases. but usually you would speak a question with a different tone in your voice then an order, right? that would be enough to differentiate. i mean then someone is talking to you hear it in his voice if something is meant to be a question or if it is meant to be an order.
for "do they speak english?" you wouldn't capitalize the S in sie!
Sprechen Sie Englisch? - > do you speak english? (formal)
Sprechen sie Englisch ?-> do they speak english?
since he wrote it with a capitalized S in Sie it is clearly "do you speak english?" not "do they speak english?"