About [[FreeCol:Flag.background.FESSES/en]]
Fragment of a discussion from Translating talk:FreeCol/LiquidThreads
Based on Flag.java, this is supposed to refer to horizontal stripes. I am not familiar with this usage of the word fesses personally, but according to the information cited below, it may also be used on the coat of arms.
From the Merriam-Webster dictionary:
Definition of fess 1: a broad horizontal bar across the middle of a heraldic field 2: the center point of an armorial escutcheon
You can subsequently find more about a fess on Wikipedia.