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This already works. LoginForm::displaySuccessfulLogin() always has the logged on user name as a parameter and uses $wgOut->addWikiMsg which processes magic words.
Only("You are now logged in to translatewiki.net as "$1".") is fed into displaySuccessfulLogin() in core and svn extensions.
Looking at the translation hint, that's been in since 16 March. What do we have the hints for anyway of they are not read?