This achievement is obtained in Expedition mode and in order to get this, the Virtual Pro's nationality (VP) that you pick at the start of the Expedition mode will determine which country the players need to be from in order to unlock this achievement. You might be lucky and get some random players at the start of the Expedition mode, but you will need to unlock the national team during the course of the expedition mode, which your VP is from, and play them 10 times maximum. You will get a player each time you beat them and you will have to rinse and repeat until there is a full team from the same nation.
For example, my VP I picked was Steven Gerrard who is English, I didn't get any random players from the same nation. I unlocked playing against England about 6 games into the Expedition mode, however, this can be shortened depending on which group you chose your team to start off in. I just need to play England 10 times and beat them, fill my team with the 11 and then win another Expedition match, and then I shall have the achievement. Please note that it all depends on the nation your VP is from.