Poland is dead easy - not a bad idea to have a letter from the gun manufacturer to state that your gun was manufactured as a sub 12fpe gun and if you have some form of invitation to a shooting event that'll be good to.
When I went to Poland in 2006 with two rifles, no one ever asked to see any documentation and only once on the outbound flight from Stanstead did anyone even look at the gun.
France.. they've just recently changed their laws to make carrying sub 20 Joule guns through france much easier. Again, no need to get a firearms cert, but if you've got a couple of letters that'll help you if you get into hot water.
Portugal, I don't know for sure, but I believe you don't need a license for a sub 12fpe gun out there either.
Germany - avoid!
Major pain in the bum