In a recent interview with The Times of India, Spielberg gave an update on the film saying they have completed the script.
Peter Jackson is directing the next one, I'm producing. We have a script and we're going to start performance capture probably at the end of this year.
Don't hold me to it, but we're hoping the film will come out around Christmas-time in 2015. We know which books we're making, we can't share that now but we're combining two books which were always intended to be combined by Herge.
This goes right along with what Jackson said a couple of months ago. The last I heard, the sequel will be called The Adventures Of Tintin: Prisoners of the Sun, the script for which was written by Anthony Horowitz. The plot of the film involves Tintin and Captain Haddock investigating a strange illness that has befallen seven explorers who have discovered an Incan tomb, before traveling to Peru to get to the bottom of the mystery.