Guilds and Nationality

Simple enough.

If I change my nationality from American, to European or Japanese, or whatever, will it effect my membership within my guild?

Ei. If I change from American to European will I automatically be removed from my american guild?

No, you can have americans in european guilds etc. It's the majority that counts, or the founder of the guild I think.

if you've watched the Fire of Spirit guild's video I think, several members are from various places

You'll just play in International District if you PvP with different nationalities.

Nationality doesn't matter, we have people from all over in WPG.