Thinking in 'mother tongue'

I have heard many opinions, read many articles about the importance of learning in language that 'you are brought up with, if not born with'. Thinking does not have any language. Doesnt a hearing/speech impaired think? Probably having studied Mathematics atleast upto graduate level, I tend to think that we think in 'symbols' and 'equations' only. One can name the variable as x or க or λ or क, but that doesnt alter the equation, does it? Language comes into discussion when we think about EXPRESSION OF THOUGHTS. So we dont think in 'mother tongue' but we may express better in 'mother tongue'. I really would like to know what people MEAN when they say that we THINK in our mother tongue.