Interesting. Italian has 'americano', which is sort of the common way of putting things, even if not entirely correct. 'statiunitense', similar to the spanish word you cite, is the more formal term. It is however, an unwieldy word that's mostly limited to news or official things, and 'americano' is what I am to most people here.
The more informal Spanish version would be "norteamericano", which also packs some odd ambiguities, since it is mostly a reference to people from the US.
The bit I was digging into there, by inference (not necessarily clear for those not familiar with Spanish) is that because North and South America are taught to be one continent in Spanish, and "americano" references the peoples from that aggregate, Spanish speakers (mostly Latin-Americans) have a habit of correcting English speakers in online forms when they use the word "American" to refer to people from the US.
