Well i know the mono version was being mixed on the opening day, May 25th, since Lucas tells the story of how he went to go to the MGM mixing studio to work on the mix and noticed the lines around the block and then phoned up Hamill to come down and loop more dialog that very night. I would assume that the foreign mixes would be done after this. IMDB lists opening days for overseas as much, much later in the summer, October, November and December for many european countries (although this could easily be inaccurate, i would think not by a whole lot).