Most alien (or non-human of any kind) languages in media are simplistic and based on the language of the creators of the media. Provided the languages have some form of grammar established, be it languages from fantasy creatures or aliens, they will always look more like English than even Welsh looks like English. It seems that even when aliens aren't speaking English, they're speaking something like it. In the Con Lang community, these alien languages would be described as a "relexification" of English, or relex for short -- many of these may count as fictionaries.