Now a days the most important thing for getting accepted in a campaign is the post quality... you might have a sr. member account but if your post quality is poor then it would be hard for you to get accepted in a good campaign.
it is not a problem in most cases, if anyone knows english language pretty good, he won't have that problem right?
I don't know maybe if someone knowing english will not be counted as a spammer but i don't think that is how it works because mostly campaign managers see the quality of your posts but not your grammar...
Yes Grammatical mistakes can be avoided and the basic purpose is to contribute to topic by posting quality post which will be valuable to other users on the forum.