The server population is totally unbalanced, most of the players (and almost all higher level players) have chosen Spain or Portugal. Basically all players in the existing 3 guilds have chosen Spain and Portugal.

Similar naval MMOs have the same issue, for example Netease War for the Seas.

Servers with a one-sided server population will die a slow death because there is no competition. Just take a look at War for the Seas. Trade wars to control ports are an important part of the game but when Spain controls almost all important ports Spanish players will become bored and non-Spanish players will be frustrated and just leave.

There are two ways to address this which have been tried and tested in other faction based games:

a) Lock overpopulated nations so no new players can choose those nations

b) grant SIGNIFICANT ducat/exp bonusses to low population nations (currently Holland, France, En*****) to give players an incentive to switch to those nations

Players can switch nations but are generally lazy and very hard to convince to change nations. 

Also please nation lock guilds. cross nation guilds make no sense at all under the premise of the game. Trade wars are nation based.