Thanks, folk! I had made a patch along the lines of Bryan's suggestion more or less, making the format non-static and initializing the db locale in the contructor. Knut's idea seemed attractive, however, and I made a try. It turns out the MessageBuilder chokes on it when it tries to generate DITA source, not being prepared for anything but
parameters in message strings in messages.xml: it tries to put the name of the meta-variable in the string, cf this:
<entry colname="col2">Privilege not revoked from user <varname><authorizationID></varname>.</entry>
Obviously, trying to format a string like "<varname><number></varname>" with the format "
" doesn't work very well...
I'll see it I can work around that somehow.