Relex

So you have an alien species or a fantasy nation in your work of Speculative Fiction. For some reason you need to invent a language for them, for names or some ritual phrases if for nothing else. You realize that it makes no sense if they just spoke English, so you make up some words for the language you are inventing, and then just take an English phrase and substitute your made-up words for the English ones. Voilà! Here you have a phrase in your very own Con Lang! Right?

Well, it may indeed be considered a Con Lang, but only the most primitive kind, called a "relexification", or a "relex" for short, and hardly any more realistic than if your fictional people just spoke English. What you just created is a simple cipher for English. Substitute English words back for your made-up words, and again you have an English phrase that makes perfect sense. In the real world, that wouldn't be possible even for very closely related languages.

Indo-European Alien Language is a step above this, when the author actually invents a language with its own unique grammar, but which still follows the typical patterns of Indo-European languages. Truly original creations fall under Starfish Language.

Conveniently Precise Translation is a direct result of this trope (or, at least, implies it). Cypher Language is when this happens to the language's writing system.

Remember though, Tropes Are Not Bad. The easier a language is to translate, the more fun can be had with hidden messages and wordplay.

Films

 * The bits of Vulcan we hear spoken during the opening scenes of Star Trek IV: The Voyage Home appear to be a simple relex of English -- and not a terribly sophisticated one, since the word for "logic", used several times, appears to be the near-anagram "olgica".

Literature

 * In Artemis Fowl, the fairy language, including its writing system, amounts to a direct cipher of English, despite the fact that it's out right stated that the (very different from English) ancient Egyptians derived their language from the fairy language.
 * The Inheritance Cycle has an example of this, with a language for each species, although the human one is never specified with any differing words, presumably Translation Convention. The most well-developed language is the Ancient Language, based on a combination of English (grammar-wise) and old Norse (some grammar and vocabulary). The English relexification shows in a few places, most notably in the plot point differentiating between "shielded" versus "shield" (the verb). There is no language in the world that forms the transitive past by adding the past tense morpheme to the noun form of the verb—however in English, the present transitive and the verbal noun look the same, which is where the confusion arises.

Video Games

 * The Dragon language in The Elder Scrolls V: Skyrim is basically a relex of English; the script for this language, however, was invented from scratch. (Literally. The script looks like it was scratched into stone by dragon claws.)