Should be able to use GENDER for the username, for example necessary in French.
I think this might already work. LoginForm::displaySuccessfulLogin() always has the logged on user name as a parameter and uses $wgOut->addWikiMsg which processes magic words, as far as I know. Nike could confirm...
Only("You are now logged in to translatewiki.net as "$1".") is fed into displaySuccessfulLogin() in core and svn extensions.