The ten millionth article has been written on Wikipedia – a Hungarian biography of of 16th century painter Nicholas Hilliard (English version here).
Those ten million articles have been written across 250 different languages, Wikipedia says. English is still the most popular language on Wikipedia, with 2.3 million articles (they reached 2 million English articles in September 2007). After English, the next most popular languages are German, French, Polish, Japanese, Italian, Dutch, Portuguese, Spanish, and Swedish.
See here for an article count by language.