I do not think you are doing anything wrong here. Try to convince your husband that, why he has not learned Tamil do others and not you ask a question. When you are comfortable with situation why he should feel offended. Moreover, to your friends, you may tell them it is well-understood issue between two of you and there is absolute comfort on that. Love is beyond the limitations of language. Another interesting fact I want to share with you is, females learns language faster then males at any stage of life.