Your mother tongue is the language that you learn when you are a baby.
近义词
native tongue , first language
双语例句
例： The islanders speak English, but their mother tongue is Gaelic.
该岛居民说英语，但他们的母语是盖尔语。
例： A truly bilingual person has not one mother tongue, but two.
真正通两种语言的人不是只有一个而是有两个母语。