The French flag was the national flag of France.
Behind the scenes
In Pirates of the Caribbean Online, a modern French flag is displayed on the French privateer island Ille D'Etable De Porc, instead of the one from the 18th century. This could either be a mistake by the developers or it could even be Pierre le Porc's personal flag. The flag he uses was not used by France until the French Revolution (1789-1795) when the monarchy was overpowered by revolutionaries.