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 :)
この言葉は特に、二つ以上の言語が話せることについて言う場合に使われます。例えば：
"I speak Spanish but my first language is English."
（私はスペイン語を話しますが、母国語は英語です）
お役に立てば幸いです。
My local dialect is Genovese, which I spoke first and foremost as a child.
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.
子供の頃家庭で学び話す言葉が 'native language'(母語)であることは明らかです。しかしいつもこれ程明確に言えるわけではありません。
国全体では、公用語を話すかもしれません。例えばスペインではスペイン語です。しかしカタロニアのような地域には、その地方独自の言葉があります。
もし子供の両親が日常的にカタロニア語を話すのであれば、その子供の mother tongue (母語)はカタロニア語になります。
もし片方の両親がスペイン語を、もう片方の親がカタロニア語を子供に話す場合、おそらくその子供は幼少時からバイリンガルになるでしょう。
First and foremost は、「まず最初にそしてもっとも強く」という意味です。
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.
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'
「母国語」はfirst languageとかmother tongueで表現できます。
「母国語」の直訳はmother tongueですが、
first languageも「第一言語」の意味なので、同じものを指します。
Her mother tongue is Spanish.
「彼女の母国語はスペイン語です」
以上です。
少しでも参考になれば幸いです(*^_^*)
イギリス 
ドイツ 
アメリカ合衆国 
日本 
