The Basque language - which is spoken in the Basque Country, and also extending over a strip along eastern areas of the Bay of Biscay in Spain and France - is classified as a language isolate. There are currently around 800,000 speakers.

It has no demonstrable 'genetic' relation to any other known language (with the exception of Aquitanian, which is thought to be an ancestral form of Basque)

There are written records from as far back as the 11th century, but its origins prior to this are generally considered unknown.

Basque has been hypothesized to be the last remnant of a pre-Indo-European language family once spoken widely in Western Europe.

Source : Wikipedia

For extensive technical details, see Basque and the Reconstruction of Isolated Languages Lyle Campbell, Routledge Handbooks, 2017