Should your mother-tongue necessarily be your first language?

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So I have been a member in this site for a number of years doing surveys and getting paid with no problem.But recently ,I received a message from the Admin that my account is under review and so my payment has been suspended.It was also included in the message that I should update the pre-screening responses which were incorrectly filled. I believed all my responses were genuine and so didn't bother to change anything at all.Then I received a message that my account has been banned for lying that my first language is language is English. I have always preferred English as my first language because its what is used in all learning institutions and am not even fluent in my mother-tongue.Then we have 'Swahili' language which is often assumed to be the first language in my country yet to many its for those in urban areas due to the many tribes that we have. But anyway my account was reinstated after I sent a message explaining my thoughts on this issue of the first language.