The first sentence works. Polishing it a little would make it sound more natural, though. 필요한 것은 없으세요?(or 필요한 것 있어요?) 저, 시장 갔다 올게요. - and actually, you would usually put 저 시장 ~ first than 필요한 것은 ~ . The second one, I don't get what you mean clearly, the English version either. Do you mean "I'm not sure if I can speak in Korean or not."? If that's the case, your example would work, but it's slightly awkward.