Tileira:
From my experience, if someone is applying to play in a game in a secondary language, they are more careful about their grammar and spelling than a native-speaker.
The only players I have met whose English has been so bad I have refused to take them have English as a "first" language: the people who don't punctuate, don't use capitals, and expect you to take up the slack.
(This comment excludes dyslexia and other issues and only includes the people who blatantly don't care to try).
English is not my first language (I'm italian). I know I make errors. I usually warn people about the fact english is not my first language (both in RTJ and in Game Info presentation as GM)
Sometimes I receive compliments for my english, sometimes I make very stupid errors. It doesn't matter. I make and extra-effort to play and lot of people recognize it. In the same way I do not claim for special treatments.
I have to say there are lot of people english speaking as first language (I suppose kids) that use a lot of slang, bad spelled words and terrible punctuation.
It has to be considered that people that talks more than one language usually have an higher cultural level than who doesn't.
Of course I would perform much better roleplaying in italian but there's no so much activity in italian language.
However, no reason for anyone to be unpolite.
P.S. I've meet lot of people claiming to be able to talk italian on the web. They obviosly cut and paste from Google Translator and the result is a disaster. LOL. Talkin a second language is not so easy. Everyone should be encouraged in that effort.