To me mother tongue is from my mother land which is the present day Khurasan Razavi Province in Iran. The mother land of the Hindus is Brahmanabad-Mohenjodaro and Harappa, which is in Pakistan. Ganga and Ayodhya came later.
i have heard two definitions of mother tongue: 1. The first language learned by a person 2. The language one thinks in i read a brilliant article recently in the annual yaadgaar e raza about alaHazrat's mastery of the arabic language. it had a quote from alaHazrat where he wrote [rephrased]: i think in arabic and when i write in another language; i have to effectively translate. no wonder that he wrote 300 works in arabic.