English is my native language - the grammar and idiosyncrasies are so ingrained now that I find it difficult to pick up another language (I suppose immersion would help). I tried Latin, French, and German in school and never could get comfortable with any of them (I got close with French). A couple of years ago I tried to pick up some Japanese (I work for a Japanese company) but it just didn't stick. I find Japanese culture fascinating, though, so I may get back to it eventually... probably just in time to retire.
Now all that useless trivia being said, I think it would be a great idea to host PbP in foreign languages. The RPG phenomenon is a world-wide craze, not just an English-speaking one. So, the more the merrier indeed!