There's only one change in version 1.3.1, the missions are now also available in Russian too. The translation was done by Warhamm.
Everyone who already had the pack installed previously and now wants to play the missions in Russian, has to delete the 'mtreecache' file in the 'IL-2 Sturmovik Battle of Stalingrad\data' folder. The 'mtreecache' file saves the mission names and briefing texts. When the game is started after deleting it, a new file will be automatically created and the Russian briefings should now be visible.