Language border

geolinguistic boundary between mutually intelligible speech communities
Borders between German, French, Italian and Romansh in Switzerland

A language border or language boundary is the line separating two languages. The two languages must be different enough for speakers to not understand each other. If the speakers do understand each other, there will be a dialect continuum with no exact border, and the languages will mix with each other.

A language island is a language area that is fully surrounded by a language border.