The MessageBuilder program turns messages.xml into two output files:
1) The DITA source for the Reference Guide section on error messages.
2) The messages_en.properties file of engine messages.
All of the translators I have dealt with want to use (2) as the English input for their translations. I believe this is because translators have written their own tools and those tools expect to work on properties files.
Messages may have arguments. Those arguments appear as sub-elements of the messages in messages.xml. The arguments turn up in (1) but are thrown away when MessageBuilder produces (2). We can help translators do a better job by also writing the arguments as comments in (2).