I would personally think that, if they're using magic to get around the language barrier, it works perfectly. Their grammar is impeccable, probably more strictly correct than that of the native English-speakers they're talking to. Having major and supposedly intelligent characters communicate in poor English throughout the book is just a needless annoyance for the reader, unless you do it masterfully (which almost nobody has ever done). They probably speak in a very precise but slightly stilted way, a lot like anybody from a Jane Austen novel who wasn't working-class. Here's how to make them alien. Some things simply don't translate, so every so often, the construction of a sentence is just plain wrong. The example that springs to my mind is a Tim Powers novel I read ages ago, the title of which I forget, in which the hero interacts a lot with elves, humanoids who have a totally non-human mentality which the reader never really gets to grips with. At one point, their leader says to him: "We have decided to like you." See what I mean? That's absolutely perfect in terms of being subtly alien! No human being would ever speak that sentence unless they were a grinning cult weirdo you probably wanted to run away from screaming. But for elves, it's the literal English expression of what they feel. There's no backstory explaining whether they have a telepathic hive-mind, or hold committee meetings to decide everything, or maybe have insanely detailed rules governing social interactions written down in million-year-old books. Maybe all or none of the above. Who knows? They're just being elves.