Recently, I've had several enquiries about the Language component in GP, mostly regarding the basis on which we assign the grade. Without having to explain the entire "band" marking scheme which can get unnecessarily quantitative, here's my own qualitative approach to gauging language use in a paper:
You have to imagine your essay or compre script as a discussion between yourself and your examiner. You are both seated at a coffee table talking over the issue of your question. The examiner is most likely to be a native speaker of the language, and you are, well, you.
Ask yourself how comfortable you are with the situation. Can you have as easy and fluent a conversation with your examiner as you could with a friend? Is what you are saying comprehensible to your examiner or are you likely to get lots of "excuse me," "pardon me," "could you repeat that s.l.o.w.l.y. for me, please" requests? Are you giving the impression that you are treating the examiner with the proper respect as an equal, or -- like you do with your mother -- are you trying to brush off the issues as quickly as possible so you can get on with the rest of your life?
You can multiply these self-check questions with the application question. Sitting with you and your examiner are the authors of the articles that you are reading as your compre passages. The AQ is a full-blown discussion amongst the 3 or 4 of you, all learned, articulate men and women, interested in the world, sharing different ideas from all your myriad experiences over biccies and a hot cuppa'.
The ability to speak a language is a mark of peerage. In other words, if two people use exactly the same language, they are equals. In this case, the more equal you are, the higher you're ranked. Paradoxical, but there you go!