shield on back = never affects your damage or attack speed with ANY weapon. The weight can slow your movement speed though.
Langes = is a 1h weapon but can be 1h or 2h. If you are carrying a shield on your arm, it will be 1h. If you have no shield on your arm, it can be 1h or 2h. If you have a shield on your back it can be 1h or 2h. There is never a penalty with this weapon.
Some 2h weapons (longsword, bastard sword, etc) = can be used with a shield on your arm. There is a bad penalty for that. If there is a shield on your back there is no penalty.