I agree with Pierre. The English title of the film is A Very Long Engagement, and, even though it is based on a work of fiction by Sébastien Japrisot, it manages to be more believable than Joyeux Nöel.
I have seen Joyeux Nöel twice, once in the theater and once on DVD, and, though its heart is in the right place, it gets the history so wrong it's pathetic. The biggest of several inconceivable gaffes occurs when the Bavarians, now apparently a Strafbataillon, are sent, via cattle boxcars, to fight the Battle of Tannenberg in the spring of 1915 (when that battle had already been fought between 26 and 30 August 1914).
My personal favorite of the recent cinema of the Great War is François Dupeyron's La Chambre des Officiers (2001).
Gillies MacKinnon's Regeneration (1997) and William Boyd's The Trench (1999) are also excellent.