In a recent blog post from Johannes Schluter he proposes a "universal language" for documentation and code comments - English.

The argument for using only English is simple: Every developer should know at least basic English and using English helps when either outsourcing, opensourcing or reaching new markets. Now there are arguments against English, too, one is that not all developers are able of using English properly so reading and writing comments is more difficult for them - now you don't have to write long prosaic texts in comments, but still...

Comments on the post vary - some disagree and some say that this is how they're already doing things and it seems to be working out well.