Here's how (Mac --- same may work on Win)
Edit the file:
/Library/FileMaker Server/Web Publishing/publishing-engine/jwpc/tomcat/fmi/VAADIN/themes/default/i18n/en
(Strange that I didn't think of looking there in the first place!)
USER_LOGGED_OUT=%s, you have been logged out successfully from '%s' database!
(I added a space after the '=')
Restart the WPE -- voila!
OS X Find does not penetrate these depths, it seems, but by putting copy of the 'Web Publishing' folder on the dektop you can search that and find quite few goodies.