The nationality has no discernable affect on the player's performance. I have a good Ghanaian! I like collecting less common nationalities.

MC's and playmakers can be hard to get the best out of.. if I had him, I would add to his physical column to get it as much or more as his attack, and play him in as many friendlies as I could, trying arrows, position, who he plays with, to see where he does best. I am happy when my MC's get a 7, though. 9's out of them are rare on my team(s). If after trying everything like that, if he is still not producing for your team, sell him.