This cheat is to unlock a certain nation team's all star team, I currently found this out for Argentina. Enter an International Cup mode and win it, simple as that, then you should get a screen telling you: "You can now use Always Argentina" (or whatever team you choose to enter the cup with.
Unlock all Cups and Players
Get all cups and players in start screen press Up, Up, Up, Up, Up, R2, R2, L2, L2, R1, L1, Left, Down, Right, Up.