The move is the second stage of a push to get Twitter into as many languages as humanly possible. I say humanly intentionally as the entire translation process is being open sourced.
Twitter appear to be working round the clock these days, with two solid updates in the last 24 hours, the first being the most significant, with Twitter providing geo-location access to developers via the API.
Nevertheless, this is bound to bring a smile to the thirty French speaking countries active on Twitter. To see Twitter in French, visit your settings and select “French” from the menu.