が and は are grammatical markers (particles) that indicate the function of nouns. I am not quite sure whether bahasa indonesia has this feature, but it seems to be somewhat unique so it might not make sense at first.
は is a general topic marker, and is used when talking about a general or neutral fact, e.g., "English is important".
が is a topic marker that is used when the topic has already been introduced, or the reference to it is clear. In your example, it is clear that the topic is the speaking of English. The question is more like, "English, do you speak it?"
BTW, there are numerous other markes that perform different functions. I hope this make sense.