In tag questions, the verb in the question tag is the opposite of the verb in the first part. IT IS raining, ISN'T IT? YOU'VE been ill, HAVEN'T YOU? like that. . . So, "English IS your hobby, ISN'T IT?"

Both are correct but there is a subtle difference: "English is your hobby, isn't it?" implies that you already know the answer and are looking for confirmation, while "English is your hobby, is it?" is more of a direct question.

A hobby usually refers to an activity of some sort so, 'Is speaking/learning/practicing English your hobby?" would work better. "Learning English is your hobby isn't it?' "Is practicing English one of your hobbies?" 'Hobby' isn't really needed either as you could just ask, "Are you Learning English?" or "Excuse me, that sure does sound like English you're speaking" hahahahahahha