My suggestion is hunt down someone who is a native speaker to play through your game. If it is really bad, it probably won't fair well compared to other entries, but if it is coherent and not terribly noticeable I don't think it will make that much difference in the grand scheme of things.
I have the same problem. It's not so easy to find a native speaker who is willing to do so in such a short time ...
haha yeah when i told my friend the contest will only takes about one month he say "Oh no" XD
maybe i will make short conversation game next contest XD i just joining the contest so i wont leave my project and go to other project