Hmm...it's a personal opinion, but I think that online chat rooms or multiple choice grammar test won't help you much outside from grasping the basics of a language. And a crowded chatroom actually punishes people who want to write long sentences (eg: if I had written this sentence in a chat room, the part I'd be replying to would be already be pushed off the screen). You'll probably be better of reading books (any book is fine...even a comic book if reading isn't fluent enough yet).
And I'll be honest: if English isn't your native language, it's A LOT harder to get the knack for it. I mean...I'm Dutch myself, but even though I've been spoonfed English through film/television since I was young(1), actually conversing in English through forums also took quite a while to get anywhere decent(2).
But to get back at writing: does it really have to be in English? As my nickname suggests, I love writing. But while I can make myself understandable in French, it's just...not fun. Depending on the subject, writing isn't an easy thing to do even in your native language. And if you do not just want to write anything but an actual game, then I would certainly advice to just write it in your native language and then translate it (or have it translated or corrected). That way you can focus on what's important rather than dabbling into what is, in the end, a distraction from your goal (a tangential objective at best).
Oh...speaking of which: that book I talked about earlier is
this one. I haven't read it so can't comment on it, but I feel that reading that will help you more than something like Renate Dorrestein's "het geheim van de schrijver"(3) or Nathalie Goldman's "Writing down the bones". These are good books for sitting down and scribbling down a page of text, but I honestly don't know if that approach works on video game writing projects.
It's an example, but I see what you mean.
@Longshot56 : just out of curiosity: why not Chinese? If that would have been his mother tongue, it would've made much more sense.
(1): we don't use dubbing here, so it's perfectly normal for us to read subtitles
(2): I've been doing it since...erm...2004-2005'ish. So yeah...you're looking at about 14 years of practice. But if it's a comfort: I've written map reviews after about a year, after someone told me my writing style was better than most of his fellow Americans
(3): which is afaik never translated outside Dutch