Bruno Faidutti wrote an editorial on his website about the language in which he wrote the rules of his prototypes.
Basically he says that "I’ve even found out that even French publishers prefer English language prototypes, since they make easier to playtest the game with foreign publishers".
This is certainly a good point. But I think that there is another good reason: it easier to involve foreign playtesters.
I like to have the chance to send mine prototypes to a complete different gaming group, possibly in different country.
I remember that for Oltre Mare in the first version I sent to Henning Kropke in Germay. He planned to visit Mik Svellov in Copenahagen for some gaming sessions. So the proto of Oltre Mare received some playtests in the north of Europe before to be released. I had also a good preview from Mik in his website brettboard.dk before the fair!
At the moment I have a prototype who was sent to the Gathering of Friends and hopefully will be playtested. Moreover I will attendee the Bruno Faidutti's Ludophatic Gathering during the next weekend. In both case an english rule is a good point.
I have to admit, anyway, that the language in which I write new ideas that come out from playtesting sessions are just in Italian. I found easier to formilize these ideas in my native language. At least for the moment.
It maybe also depends of the fact that I usually share these ideas with my gaming group members that are Italian.