From what I've seen of your writing on this board, your english skills are more than good enough to get you through nuke school. (for what it's worth, I am certified to teach english as a second language by Cambridge University in England)
As for how hard it is, well....it's set up on a kind of curve so that everyone ends up working just as hard as the next guy. Sure, there are a few genius exceptions in every class, but they want the maximum out of each individual so everyone gets pushed to achieve that.
I was in class 8103 when they had school in Orlando and was placed in section 2 (dumber than a box of rocks, as we used to say). While we were working our way through algebra, for example, there might be guys in section 12 (rocket scientists) doing differential calculus. It's hard for everyone. Like thompson63, I feel that this was the most difficult school I've ever been through. But it was one great education and combined with my experience in the fleet it opened many doors later on in life both in and out of the nuclear business.
Anyways, back to the english. Don't sweat it. What writing you do will be more technical in nature for survey maps, progress reports, etc... And the navy has its' own way of communicating that you will learn along the way.
Personally, I think the Navy Nuke program is a great way to go. Whatever you decide, good luck!