First, to mod a nation, back up these files:
dutch.xml (Dutch, in your case) (location: C:\Program Files (x86)\Steam\steamapps\common\Rise of Nations\tribes) and
help.xml (location: C:\Program Files (x86)\Steam\steamapps\common\Rise of Nations\Data)

After youve done that, edit dutch.xml on something like notepad. find <TRIBE name =""> and replace the 'Dutch' with whatever you want your nation to be called. Dont remove the "" though, cause that will create an error. You an also change whether they are an Asian, korean, european,, mediterranean, South american and the cities that will appear when you create one. Then go to help.xml and look for "The Dutch have the power of Commerce". Replace that with "The nationname have the Power of power" e.g. The Swedish have the power of the Baltic.
example custom spanish.xml that now is actually the Swedish (by me):
<TRIBE name="Swedish">
<LEADERS>
<LEADER>King Gustavus IV Adolphus</LEADER> leaders e.g. Vansickle
<LEADER>Louis Gerhard De Geer</LEADER>
<LEADER>Gillis Bildt</LEADER>
<LEADER>Nils Eden</LEADER>
<LEADER>Carl Bildt</LEADER>
<LEADER>Stefan Lofven</LEADER>
</LEADERS>
<CITIES>
<CITY>Stockholm</CITY> the cities e.g. your home town
<CITY>Trondheim</CITY>
<CITY>Uppsala</CITY>
<CITY>Sundsvall</CITY>
<CITY>Trelleborg</CITY>
<CITY>Helsingborg</CITY>
<CITY>Vaxjo</CITY>
<CITY>Orebro</CITY>
<CITY>Borlange</CITY>
<CITY>Gothenburg</CITY>
<CITY>Vadstena</CITY>
<CITY>Visby</CITY>
<CITY>Jokkmokk</CITY>
<CITY>Storuman</CITY>
<CITY>Gavle</CITY>
<CITY>Sama</CITY>
<CITY>Umea</CITY>
<CITY>Ostersund</CITY>
<CITY>Lulea</CITY>
<CITY>Harnosand</CITY>
<CITY>Hudiksvall</CITY>
<CITY>Linkoping</CITY>
<CITY>Jonkoping</CITY>
</CITIES>
<UNIT_CONTINENT>0 European</UNIT_CONTINENT>
<BUILD_CONTINENT>1 NEuro</BUILD_CONTINENT>
<BACKUP_BUILD_CONTINENT>0 Med</BACKUP_BUILD_CONTINENT>
</TRIBE>

As you might know, I do this stuff all the time.
Have fun editing!