I just did an update to one of my Debian Wheezy servers. After the upgrade Mysql failed to start. That sucked, but fortunately I was able to fix it.
All I needed to do was comment out a line from /etc/mysql/my.cnf.dpkg-dist.
If you run into the issue, the problem is related to this line in my.cnf.dpkg-dist file.
lc-messages-dir = /usr/share/mysql
I commented that line out and the Mysql server fired right up.