There's a wide, and easily detectable, gulf between someone who's native language isn't English and someone who grew up in an English-speaking country but can't be bothered to put a decent sentence together. The difference between a native English speaker who can't be bothered to write well and one who is just a poor writer is narrower, but (I think) usually still detectable.
Really crappy word choice, spelling, and syntax, embedded in a document that presents ideas in a clear, complete, and logical order is a lot easier to read than perfectly spelled, impeccably constructed language that can't keep to the same thesis from one sentence to the next.