The sidebar uses mainpage-description now, so we need values for accesskey-n-mainpage-description and tooltip-n-mainpage-description. These should be the same as the values for accesskey-n-mainpage and tooltip-n-mainpage, respectively. The old messages probably need to be kept for compatibility.
This is a regression from bug 8617.
See [[MediaWiki talk:Accesskey-n-mainpage-description]] on enwiki for my analysis of the situation there. It's just a bit complicated...