It may sound a bit odd, but perhaps a point of clarification is needed.
English is my primary language but not my mother tongue. What is the difference? It is my strongest language but it is not my mother tongue. I was born and raised in Hong Kong which back then was a British colony. Throughout my entire education, all subjects were taught in English except 2 subjects, the Chinese language and Chinese History. Therefore, my book is written in English and not Chinese as I can express myself the best in English when it comes to writing (The Chinese translation is still under consideration).