In order to be considered Celtic, the Celtic language must still be spoken there. Over time, this language is heard of less and less. Instead, people speak English. The six Celtic nations include Scotland, Ireland, Isle of Man, Wales, Cornwall and Brittany. That doesn’t mean that all of these places have everyone speaking a Celtic language.
Instead, it is spoken to some extent and some areas in that place. In Ireland, the Celtic language spoken there is Irish. In Scotland, the Celtic language spoken there is Scottish. In Brittany, the Celtic language spoken there is Breton. In Wales, they speak Welsh which is also a Celtic language. In Cornwall, the language spoken is Cornish and in Isle of Man they speak Manx.