How should I deal with a professor whose weak language proficiency is disturbing our paper's writing quality? I am a first year PhD Student, writing a conference paper with an Italian Professor, very senior and renowned in our field. Every commit he makes to the SVN is riddled with spelling and grammar errors. I have been fixing the errors and also trying to improve the expression but I have this impression he is not very happy about me doing it. (Maybe something to do with him having dozens of publications and me having a total of Zero)

On occasion, he actually reverted my changes to stick with his wrong or inferior-quality expression. How to deal with this? I would hate to see this paper go with an inferior quality language when I could have improved it.
As a French native speaker whose English was gradually improved over the years, I've been on both sides of this kind of situation, and there are different aspects to consider: 


If your collaborator does not like you fixing typos, then there is a real problem; 
It is usually accepted that papers are not written in British-English, nor in American-English, but in Global-English, and being a native English speaker yourself might not be strict advantage. If your collaborator has successfully managed to publish dozens of papers, then either his style is somehow accepted, or all his papers have been written by others, in which case he wouldn't mind letting you write the paper in your own style; 
The style of an author is personal, and changing it can be seen as touching the ownership of the text, and can also be seen as offensive, especially when the gain of the modification is not immediately perceived (which might be the case for non native speaker, a "better" expression does not necessarily strike as an improvement). This is particularly true if you only change one expression: if you were to rewrite an entire paragraph, to change to some extent the content, it would probably be easier, since you've clearly improved the text. 
At least for me, there is a clear notion of trust of people I'm working with. There are some people I completely trust, and I don't care a second if they modify my text, but I would be reluctant to see my text modified by someone I just started working with, especially if it's only for cosmetic purposes, like picking the best expression or word when the original one is understandable/correct, or changing a notation, or reformatting the paper, etc. I'm not saying I would be against, but I would need to understand and agree with the gain. In my opinion, this is mostly a notion of trust rather than seniority. 


In summary, let it slide, as long as it does not impact the overall clarity of the paper, especially in the first stages of the paper (some expressions you don't like might disappear naturally after a while, replaced by new content).