The term 'adapt' has a couple of different meanings: 1: make fit for, or change to suit a new purpose; 2: adapt or conform oneself to new or different conditions
There's actually no such language as "Celtic". Celtic refers to a group of dozens of languages, six of which are spoken today: Breton Cornish Irish Gaelic Manx Scottish Gaelic Welsh
not sure where the official first use of the term came from by my first time hearing that particular expression was in the Clint Eastwood movie Heartbreak Ridge from Col Gunney (eastwood)