Belphegor's attack isn't "mistranslated" so much as "not translated"; the difference between the skill and the status effect 'paralyze' is lost in translation. A careful translator would've changed the skill name to avoid confusion.
The Hallucination description is a mistranslation of the JP, though the JP is unclear also. It's a status effect applied to Cybele that nullifies enemy attacks (the JP description says 'nullifies the attacks of all enemies'--not the same as disabling them!); presumably it only works if she's targeted. Nutaku also fails to convey which skills target the group and which target the entire raid. This is an issue introduced by them that didn't exist in the original JP.
Some of these problems are just careless mistakes, but others result from hiring translators that don't play the game and don't understand the mechanics, and therefore don't know how to properly convey them. Good translators don't just translate; they clarify when needed to get the intended message across. In some cases, that means rewriting.