<p>English is not my mother tongue. However, I have to write comments in English. I want to improve my "comment English" by studying a piece of code which is commented in a good English. Please recommend an open source project which contains a lot of meaningful comments written by people with an excellent command of the language.</p>

I don't think comments are a good place to learn, even if you have to emulate this style. Comments are often not even correct English and good projects will not have too many comments anyway, hence, not much to learn from.
If, on the other hand, you take a project that is commented extensively you can be almost sure that it's not a great role model since the programmers were incapable of conveying terse meaning. This is a generalization, of course. However, I believe it's (almost?) always true. YMMV.
Instead, learn English by studying the experts. There has been
a question
about this. I recommended
“On Writing Well” by William Zinsser
and I will do so again.
(By the way, English isn't my mother tongue either.)