Okay, so do you have problems with English specifically grammar?
Well using a spell and grammar check could help but these are not perfect. So here are a few pointers to help you mask your bad English.
1.) K.I.S.S. - Keep it short and simple! That's right, if you already have problems articulating what you want to say, don't do long role-plays. The shorter your rps, the fewer mistakes you make, as long as you get your point across.
2.) Play a foreign character! You can play a character that already has problems talking in English. For example, "Pardon me bad English, I was not born in America." You can get away with bad grammar, bad sentence structure with it. Wink If English is not really your first language then it would be better if you play a character from your nationality, just speak in your native tongue and insert the important parts in English.
3.) Find someone who can play a manager role for your character. Let someone who can do most of the talking and just do simple replies and rps. Or even better, play a monster who speaks only with grunts. The manager will talk, and you can grunt.
4.) Proofread your role-play! That's right re-read it again, ideally, after some time has passed, there is an edit button so you can edit your rps. Wink
Hope that helps, I'll add more stuff if I think of anything.