Firstly because countries like the US are home to many native English speaking people with non-Saxon names, and countries like Ireland in the old world speak English while retaining their native names.

Secondly because many people who learn English as a second language grow up be fluent and can write with ease. I look to places like the Netherlands where students from all over the world are writing theses for their higher education completely in English.

It's not that hard to write in your second language, certainly not to the point that your name could be in any way an indicator of your skill level.