People who support major political parties should be able to select it as an option. Currently there is a limited amount of available options. Pretty much all of the parties that you can support are American, and there are plenty of non American people who use this site.
Nationalism as a vague term has always existed with many supporters. I believe in British Nationalism where the priority is protecting English, Welsh, and Scottish culture against immigrant culture. Where one is proud of one's country. There is no racism in that, thus it shouldn't be prevented on DO as an option of political belief.
I'm am in no way anti-nationalist, but I would like to point out that nationalism isn't exactly specific enough to constitute its own ideology. Nationalism is basically advanced patriotism; you can have nationalist liberals, nationalist socialists (no connotations meant there), nationalist conservatives, et cetera. If your only political belief is that you love your country: that's great, but it isn't at all a full platform.
I do agree that DDO needs a wider variety of political customizations though.