Русско-англо-русский юридический словарь

Mother tongue and Native language have similar, meanings and are often actually interchangeable, however there is a subtle difference.

Native language refers to the language of the area the person grows up in. For example, growing up in the United States, your native language would be English. It's the language used every day everywhere you go by the vast majority of the people there.

Mother tongue refers to the language of the family you grew up in. The language your parents spoke in the house, or to each other out-and-about if applicable. Often, parents will use the same language as that of the region (parents in the U.S. speaking English) however it can differ. Take for example a family of immigrants from Mexico to the United States. In their house and with family/friends they may use Spanish. But at school, restaurants, stores, etc they would use English. In this case, the native language is English and the mother tongue is Spanish.

Typically, the two will refer to the same language, in which case they are interchangeable; however, in some cases they may differ. (StackExchange)

About 500,000 parcels are sent from the European Union to the UK every day. These will be subject to a new VAT and customs charge and will be held until customers pay the duties. (The Times)

We all make small and sometimes large personal sacrifices of our freedoms every day, as law-abiding citizens, in the interests of our fellow citizens. (The Post-Star)

After I got dumped by my boyfriend of almost 3 years we started hanging out more and more until he was over at my house every day, he called my mom "mom", we would hold hands while walking in the mall together and try on goofy sunglasses ... (Letters from the Anonymous)