On one hand, I think many of the first posts / answers are quite messy, and not only in bad English, but, and that is probably worse, the result of practically no effort to do some research before asking (or answering).
There are two very separate points there. If the problem is bad English, that is often not the poster's fault. If their English is not that good and they nevertheless put the effort into attempting to answer, there is no reason to penalize them, on the contrary, they should be congratulated. Just edit the post and improve it and vote it up if it was a decent answer apart from the language issue. Mind you, if their English is fine but they write like
Pls hlp me!!!!!!!!! im trying to isntl ubnuntu and its no working. Why??????
Then downvote to oblivion, please. We do not expect people to write perfect English but they're not texting their mates either. There is no language that allows multiple successive punctuation marks like that and we expect users to at least re-read their posts before submitting them.
Now, if they have not done any research but have just written the first thing that came to their mind with no explanation, no details and no useful resources, then that is a crappy answer, get rid of it. You should not be afraid to dowvote bad posts and flag really bad ones. Please do it as much as possible.
Think about it, the Stack Exchange sites aim to be (and largely succeed) different from similar sites in terms of quality. Allowing bad posts just so you don't hurt the poster's feelings is bad for the entire site. So, maybe the poster gets an (undeserved) warm fuzzy feeling but the rest of us have the quality of our site reduced, and the next person to come visiting will i) see that post and say "Cool, so I can answer crap here" or ii) Look for an answer and find crap.
People in this particular SE site (AU) seem to take downvotes very very personally. That's just silly. If someone downvoted you, there are two possible reasons:
Your answer is wrong/low quality/incomplete/not useful. Then the downvoter was very correct in downvoting. You should have a look at your answer and see how you can improve it. I have had very useful downvotes that helped me learn and improve my posts.
Your answer is fine and the downvoter misunderstood/hates you/is an idiot whatever. In this case ignore the downvote. Come on, not only are these useless internet points, you also just use 2 rep per down vote. It really isn't such a big deal.
Since, presumably, any downvote you give to a new answer will fall into the 1st category, you should most certainly do so. Since it's a new user, you can also leave a nice message explaining why. Here's an example of a comment I left after a downvote on an answer that was very simply completely wrong:
-1 because 1) Never parse ls, 2) this will break on file names with whitespace, 3) What is the point of piping ls to for anyway? It does exactly the same thing as the for loop alone. 4) You grep -ow does not do what you think it does, it will simply print out the first 3 characters of every file that contains three successive [A-za-z_] characters followed by a non word one, so it will also match on foo123 or 123foo` etc, irrespective of its length.
So, there is no reason to be rude or aggressive but you should be strict. That's what reviewing means. If you don't want to hurt people's feelings, skip the post or don't review. You can also flag instead of downvoting, but only do that for the really bad cases. But please, please don't give a positive review (upvote, no action needed) on bad posts to protect the poster's feelings. If that's the way you feel, stay out of the review queues. Please.