Hi there, if I ask you 'hablas español', you could answer : 'sí, estoy aprendiendo ' that would indicate that you understand some of it and that you are into practising also. You are right, if you say 'hablo un poco de español' we'd probably asume that you can barely answer with a yes or no. But if you are engaging in any conversation the other person should tell how far you could get. If you are in a shop, don't boder lol