some probably should?
Sectoid or Muton for example, are not words but if they were they would be considered proper nouns, with no equivalent in any other language, and probably shouldn't get translated, whereas Ethereal is a real adjective that describes the alien race (not as such the name of the race themselves, but a term applied to them) or Cyber-Disc which is an adjective and a noun, or Snakeman (Portmanteau of nouns), which have equivalents in probably every language and as such probably SHOULD get translated, but then there's the whole problem with culebra being a feminine noun and... well, you know how that quagmire goes. basically i guess it comes down to personal preference, i'd say leave them in the original names (in the release version) and if a given player feels an alien name NEEDS translating... well, good thing OpenXcom is so modder friendly, and those .lng files are plain text, huh?
also: "super-hombre" sounds like a MUCH cooler super hero, when will HIS movie be coming out?