I don't think a guild has a nationality, unless it recruits only those players who have the same nationality.
Most of the guild are multinational.
But if i were asked to determine the nationality anyway, For me it would be:
a) Majority of members are of that nationality.
The founder can select the nationality of a guild.
All this does iss makes that country's flag appear and the guild to appear in that country list. it doesn't stop players not of that country from joining.
It just seems like the nationality of the founder because founders set it to the same country as them usally.
But as i founder I know that we can select the country of our guilds and change them when we like using a drop down menu in the options