Loosening should allow the coolant to drain. It's a good idea to take it completely out occasionally to clean the treads and put a little anti-seize on them.
When I bought my 95 E320, the drain plug was very badly seized. Probably had never been opened to properly change the coolant even though it was serviced by the dealer since new.
I had to take the exhaust manifolds off to finally work it out. Plug was destroyed but luckily the block was OK.
If yours gets difficult to turn, be patient and work it back and forth until it can be removed.
You might need a good flush if you have that much crud in the block.