How to use NERD Commenter for vim — how to use key

Your keyboard is the problem

On many non-English keyboards, the backslash \ is only accessible through an AltGr key combination. This makes the default setting of the character rather impractical for these non-English keyboard users.
Moreover, key combinations are subject to a default 1000ms time-out. This can be observed by the disappearance of the key at the right edge of the command line. So, non-English keyboard users need to be real quick typists for \cc to work. Failure to do so, results in an undesired cc line change.
Here is an example of a German keyboard layout with the backslash-bearing key on the top row, third from right:

Solution

Non-English keyboard users are advised to change the key from the \ character to the more accessible , key. Vim beginners should also consider a slightly longer 1500ms time-out. Here is what needs to be added to ~/.vimrc

let mapleader=","
set timeout timeoutlen=1500

One final caveat; these changes only take effect after completely exiting Vim. Resourcing with :source $MYVIMRC will not work!
Done that, one can now use ,cc for commenting with NERD Commenter.