Translation:Do you speak English?
Personally, in Chinese, I ask, 你会说英文吗? to mean, "Can you speak English?"and I say 我会说英文 to mean "I can speak English."
To say "I speak English," "I do speak English," or "I am speaking English," I would say 我说英文。
However, the sentence in this exercise, 你说英语吗, could mean at least 3 things, viz.,
"Can you speak English?" as in "Are you able to speak English?" or "Do you have the ability to speak English?" or "Do you know how to speak English?" In Chinese, 你会说英语吗 is a way to specify this meaning; 会, pronounced hui4, means "can" in the sense of "to be able to." I ask the question this way; however, according to this DuoLingo exercise, 会 is not necessary, here; so, I gather that the implication is the same in both Chinese and English, when "Do you speak English?" is tantamount to "Can you speak English?"
"Do you speak English?" as in "Is English the language that you speak?" without making a direct reference to ability, or without even asking about ability. In this interpretation, the question inquires about what someone simply does in the present, or does habitually, without specifically inquiring about the person's ability to do so, or while tacitly implying the person's ability to do so. This meaning is a possibility that people may not immediately imagine, especially in the context of learning to speak a language, but an example is, "So, Swedish is your native language, but, on the job, as an air traffic controller, do you speak English?" Or, someone might ask an American Catholic priest, "At your church, do you conduct the mass in Latin? or do you speak English?" Or, say a Chinese actor gets a brief speaking part in an American movie being filmed in Shanghai; the actor's friend might ask him about his movie line, "Do you speak English?" meaning, "Do you speak English in the movie?" The actor might explain that he does speak English in the movie, even though he does not know how to speak English, except for his line in the movie.
”Are you speaking English?" as in, "is English the language you are speaking right now?" or as in, "are you presently uttering English words?" If you really want to specify this possibility, you could try 你现在说英语吗, meaning "Are you speaking English now?" as 现在 means "now."
In English, it is idiomatic to say "DO you speak English?" when one's intent is to inquire someone's ability ("can"). That is, the ability marker, "can" is omitted from speech yet understood in intent. Similarly, one asks, in Spanish, "Hablas Inglés?"
In Chinese, when one intends to inquire of the ability, of a person one encounters, if that person has the ability to speak English (etc.), is it idiomatic to include the ability marker "can" (会) (unlike in English and Spanish etc)?
British is a nationality but not a language. British people speak English.