Well, if you believe that she stuck to her score post conversati

conversation with her boss, then it is more serious. As either her boss got similar impression or she had some convincing reasons to do so. She could not just tell her boss "xxx has typos in her email so I gave her a 2." In other cases, her boss might just not pay attention at all on the review score, as most people let their subordinates manage their people unless somethings happen. If that is the case, you might want to talk to her boss (as it seems your team has conversation with her boss regularly) and present your case. You will get more impression on how serious the thing is by then. 

If you do not feel she got her boss support to give the bad review, you might give her a litle room. She could just do not expect the situation so serious and she might back off after her discussion with her boss. 

My overall impression is that you are not under the gun to leave due to this review. I could be wrong, but it is always a headach to hire new people. You are a proven asset for the company. You have been there for 10 years, you know your job and you have good relationship with your major partners. It is always risky to hire new people. They have to interview, negotiate offer, train, and might end up someone much worse. Of course, if she wants to have her own people, that is a different story, but that typically happens sooner than this. 

My sense on communication is that it is not about English, at least not mainly about it. (I know she is using that as excuse for now). I was never good at English but I was never blamed due to it. Ironically, many years ago during a review I brought it up myself on improving English, my boss said "We American just through whatever words we can think of to the email", and actually it is true. I have seen so many Americans write poorly. On the other hand, I feel one should proof reading before send out anything to avoid some silly errors. People know when it is due to ESL limitation or it is just the writer did not pay attention, and my exprience is the former is much more acceptable. 

Sorry for the long post, just my two cents.