A foreign language is a language not spoken by the people of a certain place. It is also a language not spoken in the native country of the person referred to, i.e. an English speaker living in Japan can say that Japanese is a foreign language to him or her. These two characterizations do not exhaust the possible definitions, however, and the label is occasionally applied in ways that are variously misleading or factually inaccurate.
A German student learning French.

Some children learn more than one language from birth or from a very young age: they are bilingual or multilingual. These children can be said to have two mother tongues: neither language is foreign to that child, even if one language is a foreign language for the vast majority of people in the child's birth country. For example, a child learning English from her English mother and Japanese at school in Japan can speak both English and Japanese, but neither is a foreign language to her.