母国語って英語でなんて言うの？

Mother tongue', 'First language' and 'Native language' all refer to the language of your country of origin and the first language that a person learnt from birth. All of these answers can be used as the English translation of 'bokokugo'.

This term is especially used when you are talking about the fact that you speak more than one language. For example "I speak Spanish but my first language is English." I hope that this helps :)

If we say that a language we grew up speaking, is considered our "mother-tongue".

The language that one learns and speaks at home as a child is clearly their 'native language'. however, it is not always as clear as that. The country as a whole may speak a national language: for example, Spanish in Spain. However, an area of the country such as Catalonia also has its own distinct language. If the parents of a child generally speak Catalan, then the child's mother tongue is Catalan. If one parent speaks Spanish and the other parent Catalan to the child, then probably the child will be bilingual from an early age. First and foremost means firstly and most strongly.

When someone has spoken a language from birth/early childhood then this is referred to as their 'mo 'native language','first language' or 'mother tongue'

Native language and mother tongue are pretty interchangeable, as the language that someone knows the best and is the most comfortable using. First language is more the very first language that a person has learnt to speak from birth. For example, if someone grew up in Japan and then moved to England and has lived in England for many years, they may not speak Japanese anymore and feel more comfortable speaking English. Therefore, their first language could be Japanese, but their native language could now be English.