The usage of particles is always confusing. Let me try to explain 「を」「が」「は」 Basically 「を」 marks a direct object. In some constructions, 「を」 can be replaced by a subject marker 「が」： えいごをはなす　→　えいごを/がはなせる。 ジュースをのむ　→　ジュースを/がのみたい。 まどをあける　→　まどを/があけてある。 In potential expressions like 「はなせる」「たべられる」「よめる」, that noun phrase is usually marked by 「は」 and the object of the action by either 「を」 or 「が」 わたしは　えいごを/がはなせます。 (By the way, 「できる」 always requires the object of an action to be marked by 「が」） Then if 「は」 marks the direct object, it is a topic marker not an object marker. When we use a topic marker, we exclude other possibility and put more emphasis on the item. えいごは　はなせます　Of all the languages, it's English that I can speak. So if you ask someone 「えいごを/が　はなせますか？」, you are simply asking him/her "Can you speak English?". But if you ask someone 「えいごは　はなせますか？」, you may imply that he/she can't speak other languages.

Hi Mifta! [If you say 「えいご"は"はなせますか」I think it could be wrong because it looks like Eigo is a person. And it sounds like you are asking if this person called Eigo is able to speak or not.] <- EDIT: WRONG! I made a blunder! (read the Japanese native answer down here) Instead, when you say 「えいご"が"はなせますか」 it is more similar to 「えいご"を"はなせますか」, which (I think) is correct as well. Because (with "ga" and "wo") you are marking the word "eigo" as the item that is affected by the action (speaking). I'm waiting for some Japanese native to help us. ^^

EDIT: The example looks at は and が, but the situation is very similar. consider 英語が話せますか （えいご　が　はなせます　か) and 英語を話せますか. を would put emphasis on whether X can speak or not, and が would put emphasis on what language X can/can not speak. が puts emphasis on the subject, whereas を puts emphasis on the verb. This is a problem I haven't ever seen very well explained in any textbook, but if you search, you can find quite a lot of explanations. Consider following: 私は井上です （わたし　は　いのうえ　です) i am INOUE (You put pressure on inoue) 私が井上です -I- am inoue (You put pressure on the "I") The first one could be used for an introduction, while the second one could've been a reply to the question "誰が井上ですか？" (だれ　が　いのうえ　ですか？) In the end, it comes with practice.

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.