Use language instead of lc_messages_dir for compatibility with versions before 5.5

In MySQL 5.5 language is an alias to lc_messages_dir. Before 5.5 the configuration parameter does not exist. With this change the role would be compatible with MySQL versions before 5.5 by using language instead of lc_messages_dir and making it configurable through a variable. There are still actively supported distributions such as Debian 6.0 Squeeze that ship with a version of MySQL older than 5.5.