“wa” sentence : “(you’ve asked in many languages but the guy doesn’t react to you in other languages)Do you speak Japanese?” “wo” sentence : (you just wanted to ask someone)Do you speak Japanese?” The “ga” sentence isn’t appropriate. Well, Unlike in English, that is the opposite. coz Japanese considered that the second sentence asking about the person's skills/comprehensions directly ("Anata wa nihongo ga hanasemasu ka ?") while the first sentence doesn't. There is rule involved here that isn't actually discussed until the end of course 3. This rule states that in statements where the verb is in the POTENTIAL FORM (can and can't do form) then the particle which is normally used "o" (を) is changed to GA. Of course trying to figure out why it is changed is crazy because not even Japanese people know why. It is just a rule that must be learned and followed. If this the verb in this sentence was HANASHIMASU (do speak) instead of HANASEMASU (can speak) then "o" (を) not GA would be used. This may have been confusing to you because in the question you have translated HANASEMASU as "do speak". I hope it wasn't a mistake.