Comment by ggambetta

"United States" translates literally to "Estados Unidos" in Spanish. Because some rule that sounds unfathomably stupid to me, in Spanish you can turn that into EEUU instead of EU, because both the "E"s and the "U"s are plural ¯\_(ツ)_/¯

There's a few other examples I'm aware of, like FFAA for "Fuerzas Armadas" ("Armed Forces"), but I have the feeling this is an obsolete practice.