Well in my experience, and my only source for this is Rav Shalom Shmueli and Rav Eliyahu Hemed(hence it is Sephardi practice), Tizku L'Mitzvot is said in place of Todah Rabbah. As whenever, after completeing Ulpan, I would say Todah Rabbah within earshot of one of them they would say, "That isn't Jewish, Jews say Tizku L'mitzvot". So the answer(again staying within Hebrew) could be:
Ein Bayah(no problem)
Zeh Lo Davar(it's nothing)
Mah Pitom(what's the big deal)
Depending on how proper you want to be. All of those have worked without getting me corrected for behavior unbecoming.