Once I heard from someone that your mother tongue is the language you talk in your thoughts. I've asked many people to verify the correctness of this proposition and to me, inductively this seems to be true. However, I can't deductively claim it, since I have no reference to be made.
I personally know 4 languages (Persian, Kurdish, English, and reading and listening of French). However, I think in Persian.
I just wonder, can we change our mother tongue? In other words, if we accept the above-mentioned theory for specifying mother-tongue, can we change the language in which we think? I know that I can do it deliberately, and many times I do think in English, but only when I'm conscious, and this tires me out.
However, I'd like to arrive to the point where I can think in English spontaneously. Is it possible? How? What are the techniques?