Is it true that Sanskrit is the ONLY language in the world that is called 'father tongue' while all other languages are called'mother tongues'?

Answer 1

I will not concluded that any answer is correct since we have not found all languages of the world. Moreover, among all languages found, we do not even know if they all call their languages 'mother tongues.' I would say most of the 'learned' language theorists or researchers just adopted the term from the west and use it until it is well accepted. Thai, for example, will say 'father language, mother language' as in "It is not your father language or your mother language," though most modern educated linguists may have adopted and inclined to use the term 'mother tongue' or 'mother language' in their professional settings.