Mother-tongue: complex issue
Hindi is not the national language of the Indian Union. Neither is it "rashtrabhasha" if you prefer that term. There is nothing called national language or rashtrabhasha in the Constitution of India. Whoever told you otherwise is either ignorant or deceitful. There is a way to know that. If you tell someone who says Hindi is national language that it is not, and cite the constitution and Supreme Court clarifications on that matter, an ignorant one will accept your arguments, get enlightened and correct oneself. A deceitful one will continue peddling the lie in an attempt to proliferate the idea, that Hindi is the national language of the Indian Union even after you have shown the person all the evidence to the contrary.