Actually, if you look at the word "mother tongue", I believe a better way of looking at the term would be: the language that your mother taught you from the time you were an infant. In that aspect, I would unequivocally and categorically deduce that, given the circumstances, my mother tongue would certainly and definitely be "English" and not just any "English" but "British English."
In case you haven't got the faintest idea what I just wrote, let's put it in Malaysian English: "So aaa... my mother tongue is English yaa... ok? understand? Good... Dei... let's go tapau some nasi lemak and chee cheong fun. Or you prefer to eat in? Either way boleh."
So as you can see, I was previously not able to speak Chinese dialects, including Mandarin, as I had little exposure, coming from a family which speaks English at home. It was only when I went off to Taiwan that I picked up, with some success, Mandarin, not just any Mandarin, but Taiwanese Mandarin (chei wah... action wan ah). Then back here in Malaysia, I came to realise that Taiwanese Mandarin does not jive so well with some of our local folks here, since some words are different in pronunciation. After some challenging adjustments, I learnt to convert, with some mixed success, to Malaysian Mandarin. Of course, there are some side effects to such conversion. My friends in Taiwan tell me that my Mandarin has deteriorated from their level, but at the very least, folks here have some idea what I am saying, if you know what I am saying



So there you have it! My mother tongue has always been English. I learnt Mandarin in Taiwan, and Bahasa Malaysia in school, so be kind ya, if I end up going back to English. Hard disk cannot process and translate to other language so quickly (unless, of course, if I translate to BM, which I am quite ok with). Sekian, harap dimaklumkan (That's all folks!)