Many languages can be learned even if one does not know one's mother tongue. The two have no corelation.  Not knowing mother tongue does not dilute one's love for their culture. Yes knowing mother tongue is good but can not be used as a brownie scoring point over someone who does not know it. 

If one has the ability to learn many other languages without knowing the Mother tongue, it's a blessing.

A crazy blessing but still a blessing.

We have to speak with our children in our mother-tongue. They should learn to read and write in their own language. Mother-tongue is a fundamental part of our cultural identity. It is also good to teach our children the languages of other states. This is the best way to learn about the cultures of that states. But today's children cannot go forward  without English. Therefore, we should train our children at home also to talk in English, but not always. English is the only language to communicate with the people of other countries. 

We have to speak with our children in our mother-tongue. They should learn to read and write in their own language. Mother-tongue is a fundamental part of our cultural identity. It is also good to teach our children the languages of other states. This is the best way to learn about the cultures of that states. But today's children cannot go forward  without English. Therefore, we should train our children at home also to talk in English, but not always. English is the only language to communicate with the people of other countries. 

English is the Lingua franka. In today's world to learn English is essential. But it doesn't mean one ignores his Mother tongue

Yes, we should respect our mother-tongue like our mothers. There are some parents around us who do not like to talk in mother-tongue. Therefore, their children cannot talk in their regional languages. They always talk in English and compel their children to talk only this foreign language. The attitude of such parents are talk in English is the best way to keep their status in the society.  

Talking about culture should not be restricted to mother tongue. Culture is a wide spectrum from language to clothing, to living style, to eating habit, to literature, to music, to communication pattern and what not. So, let's talk about various aspects of Indian culture and not just about mother tongue. I am in favour of learning mother tongue but sometimes due to various causes one fails to adopt one's mother tongue but knows Hindi or any other regional language quite well then I think it greatly signifies his/her awareness about Indian culture. Knowledge of mother tongue cannot be the yardstick of Indian culture. 

If one cannot talk in his Mother tongue. it's ok. But, later on  when they grow up,children will know about their Mother tongue from their parents. If parents too do not know what their Mother tongue is, then rises the problem.

Not knowing mother tongue is no longer a problem. There are many third generation persons who do not know it and are doing fine.