Program Files (x86)\Steam\steamapps\common\football manager 2012\data\db\1200\lnc\all
[username]>library>Application support>Steam>steamapps>common>football manager 2012>data>db>1200>lnc>all
Here you will find 3 or more files:
To unlock the German national team simply delete the fake.Inc file before you create a new game. The other files cause similar restrictions but apply to different competitions so you may as well delete all the files in this folder.
Note: Official updates contain these files too, so if your game has been patched then you will have to delete the files again in the relevant update folder. For example if your FM12 gets updated to 12.1 version go to > Program Files (x86)\Steam\steamapps\common\football manager 2012\data\updates\update-12.1\db\1210\lnc\all