Bornholm is an island found in the game and in real life which houses the ports of Gudhjem, Nexo and Ronne. The island, in real life, is of Danish territory, and therefore, many inhabitants speak an official dialect of Denmark called Bornholmsk.
Bornholm as seen on Google Maps
Even though Bornholm is a real island, please do note that the Bornholm island in the game is a much, much, much more simpler and more inaccurate version of the actual Bornholm, albeit the layout of the ports are more or less the same. The biggest population in Bornholm, both in the game and the real world, is at Ronne, where, according to Wikipedia, 12,887 inhabitants live.