http://www.dvdcompare.net/comparisons/film.php?fid=50198
According to DVD Compare there's a Spanish (Castilian) and a Spanish (Latin) soundtrack on the Alien 4K Blu-ray disc. Are these actually two different dubs? In what way are they different?
Spanish spoken in Spain vs Spanish spoken in Latin American countries.
Do they use different scripts or just different accents?
There are differences in grammar, slang, some words, and pronunciation between the two languages. I can't speak either so don't know how different the dubs are, but I imagine they got voice actors from Spain and voice actors from Latin American countries to do the respective dubs.
I remember quite enjoying the german dub of Alien. Ditto on the Director's Cut, though I'm assuming they hired new voice actors.