By the time the British arrived there was one language in the archipelago, Fentolese, there was always only 1 of course.
It was a dialect continuum, with at least 4 widely accepted major varieties.
Fentol, spoken in the capital and northern parts of the Fentol Empire.
Bræic, spoken on Bræ and the whole Grent Sea coast.
Aradan, spoken in the Arada Desert.
Lucinian, spoken in basically the southern third of the archipelago.
All 4 were mostly mutually intelligible.
Modern Fentolese is based on the Bræic dialect.